COLOURS  

I can help
I wrote a lot of the color commentary on that thread, if I remember rightly. I do color consultations for people who are trying to select color for their space. I'd be happy to help you with it. You are quite right to think that LRV (light reflective value) tonality and saturation will greatly affect the result. Greys can be bot heasy and hard to deal with depending on the quality of the light that enters the room. Let me know if you want help and we can correspond directly. I think 'whitespike' started that earlier thread maybe he remembers what it was called.

THANX OLIVE
The room is quite big with lots of windows .It is an L-shaped room cut out of a square .The remaining quarter is a veranda.
Although theres plenty of light entering the room and is currently painted in stupid yellow (luminus colour)The room looks awkward !
The greek marble dominates the place although it is not too dark... The heavy
veins , like dragged lines are in a darker grey than the base (light grey) but not black.It is like cement with darket grey straight lines.It looks somewhat dull and makes the whole room dull despite the yellow on the walls.
I am more and more conviced that an aanthracite colour leather sofa (cuba/cappellini will do something/ give depth.But what about walls or ceilings ?

OK, here we go...selecting color correctly takes some work
From what cardinal point does the majority of light come? N/S/E/W? Does the veranda look out onto garden/trees/greenery? What part of the world are you in? Temperate or tropical? (ie: is there a wide variation in day/night light hours?) If greenery is present outside is there a 'winter' period where the greenery is missing? Does most of the light enter through the greenery? What height are the ceilings? Are they proportional to the longest leg of the room? (roughly a 1.6 ratio) Are we truly talking a cube with a equal segment removed or are the proportions of the 'L' unbalanced? Is there a blank wall with no openings of any kind? If so is this the largest wall or the smallest? What time of day will this room be utilized most? And utilized for what? Lastly get down on the floor during multiple times of the day and look for tonality in the marble veins. There will most likely be a hint or green, blue or brown in it. Hopefully your brain is not short circuiting right now...

answers
Theres no greenery , there are more buildings. Or a few trees here and there , theres a river 300 meters away but thats interupted by a huge concrete building. I live in the mediteranean.
The room is more or less an equal L.---You take a square , and you cut out a quarter/ the veranda , then another quarter indoors you put a lower false ceiling in white stained oak
there are glass doors on the two sides leading to the veranda which faces N/W. There are additional windows facing North or West.
There is no wall with nothing on ...
All doors have windows or doors leading to the rest of the aptmnt.The height is about 2-7 meters.The big wall on the long/higher ceiling part is the one with the less mess , only the main entrance door that is again grey !
The colour that you get by looking the marble is grey-grey-grey ! I tried to imagine some blue in , but it is rather purple-ish grey.
The place will be mostly used at night hours with artificial light anyway.
There are horisontal screens on the balcony in white canvas that turn orange when the sun sets , but the room does not get any of the glow !
An idea is to go on furniture tone-on tone grey from light to anthracite (with no black or whites) and then what ? What will the walls be ? What will the artificial light be ? A warm dimmed light will look awful in such a palette.....I have a problem not only on colour but also on tone.
HELP ! ( ON COLOUR AND TONE ! )

Since you just dont like the...
Since you just dont like the grey marble?I think keying the room around greys is not a good idea you will just strengthen the fireplace.Creams and shades of white marbles seem to be more popular in the warmer countries.England is one place I know that seems to like the greys in marbles.There are some very high end designers who have painted marble fireplaces, Albert Hadley for one, but thats not an option in a rental.

My thoughts
With a N/W exposure you are getting light that shifts toward the cooler range, more blue-ish than yellow-ish. This is no help in mitigating the grey-grey-grey. Since you are in a warmer climate with a lesser shift in day/night you are getting somewhat of a help in warming up the light, but still N/W is cooler light. The buildings, especially concrete ones, outside your window are going to further reinforce this by reflecting cooler tones. So, you need warm colors in the space. My guess is that the 'stupid yellow' was an attempth at this but it failed as it's not blended well, tonally. Since you use this room mainly at night, I'd say that you should select richer colors rather than pale ones. Caramel, cappucino, espresso, butter-cream, or maybe even orange-y tones, like brick/terracotta. Don't pick anything too dark for wall paint as it will give a very high contrast with the marble and will make a room with multiple angles and ceiling heights look awkward. No matter what, you want to select warm not hot tones. Full-on hot colors, like red, will do chromatic battle with all the marble, which is probably what the 'stupid yellow' is doing. Since you imagine a faint note of purple in the grey, I might consider painting the ceiling a pale pale lavender with slightly brownish tone to it. It will balance the coolness of the marble below. This would work well with a grey-beige or caramel color on the walls. If you can paint your grey mouldings, I'd go with something that is about 50% lighter than whatever you pick for your walls. Possibly just diluting the wall color with white.
For furniture, I'd suggest rather than anthracite to go with espresso on the sofa and then bring in the brick-orange tone for accent. Stay away from blues and greens, except for in an art piece or a vase or something like that. You are also smart to expect lighting to be an issue. Too much yellow incandescent light is going to make the whole place look muddy and blah. Use some nice clean white halogens, especially as a ceiling mounted fixture that illuminates a wall. If the physical space permits, you might also try to use a mirror to catch the most western light (sunset) to bring in some warm light at the end of the day.
Do one very important thing as you figure all this out...sample and test. Get tester pots of paint, put some on all the walls and look at them at different times of day. Try different lighting plans by moving lamps around the room. If you need to, paint a whole wall to see more of a particular color. Take your time so you don't end up repainting the whole room. After the paint is on, bring home fabric/leather samples and put them in the room. Then annoy your friends and invite them over for their opinions. Good luck. Let me know if I've helped you at all or if you have any more questions.

BUT
Beige and dark brown is not exactly my taste...
Wouldn't it look like a "Minotti" nightmare ?
The beige/greige walls i'll do. Also a cool white for the ceiling.
Terracotta accents are out of the question ! It is the colour i like the least.
Another combination : Anthracite furniture/greige walls .Very masculine austere details (no black at all , no metal)and then some pale poweder pink ! Will that be horrible ? OR all the above but instead of pink , to use some Orange-orange. So tha pallete would ve :greige/anthracite/orange ....

Also copper
Can copper help ? I like the cooper shades by tom dixon , see link.
They do not only look cooper but they also reflect a cooper colour light downwards (to the floor or table
http://www.tomdixon.net/en/products.html?Gid=8

OK, so no brown.
Fine with me. but remembe that the space is quite cool and you are trying to inject some warmth. See if you can find a charcoal/anthracite that has maybe a warm hint of brownishness in it. I like the pale pink idea, but pink can be a cool color so aim towards one that has a orange-y note. Copper could work rather nicely but if you aren't careful to balance it with the anthracite you could end up with a Hallowe'en color scheme.

One more thought
I HATE beige...it is a vile life sucking color. I wasn't recommending that at all. I was thinking more in line with a yellow ochre, a greyed yellow with a warm orange-y tone. Hence caramel. What I was trying for in my earlier suggestions was to stick with stone colors, but the opposite kind of stone than the marble you've got. More like sandstone. Warm earth, but not the 'earthy-colors' of 70's kitch. Ones with a more greyed look. The 70's colors were too saturated and bright. This is where I worry about your using copper. If it's too sparkly-fresh it will look cheesy/garish with all the grey. Anthracite/palepink/greige sounds kind of nice to me, as long as the pink leans toward salmon. Like the color of a dogwood blossom. One more color to consider is the color of a smoked tea leaf. Take a look at an Assam, or Lapsang Souchong tea leaf. They are both grey and brown at the same time, it's a nice color and would be great with the dogwood pink. In my mind's eye I keep seeing a colored ceiling, how about using the pink tone up there?

white oak stained ceiling
oh yeah, meant to comment on that. Balance this by using a piece of funriture with this wood or possibly bring a rug onto the floor that mimics the look of it. otherwise you'll have an imbalance no matter what color you pain tthe remainder of the ceiling.

CEILING
You are right. I always looked up there trying to get the answer. Its on the ceiling. The walls can be a grey-beige , but without any yellow hint(as its already there and looks like a yellow wall in a grey room)
So grey/beige wall(that would blend with the floor) So we need a colour on the ceiling that will definetely reflect on the floor.The part of the room with the white stained oak ceiling, as it connects with the veranda , can be fitted with a gardenish white theme and look like a room in a room more connected with the veranda and (white panted metal furniture or white painted bentwood etc)
No countryside style, more lofty/careless.
I checked the rest of the appartments on thart same building. The nicer colour pallete was one appartment that was basically natural oak and petrol blue !Walls were grey (a bit lighter than floor) and ceilings were white .
i'm so confused...

If a picture
is "worth a thousand words". . .is anyone using computer visuals to try out different interior color schemes ?
A black and white photo or two of the space, imported into a rendering program, would enable the professsional to experiment and demonstrate, wouldn't it ?

There is an online program you can try
It's at Benjamin Moore. This might help you out. SDR is rightthat is you do a B/W picture you'll get a truer look at the final colors. I own a copy of this, I've not used the online version, but it should at least give you some idea on colors.
Note one thing from your earlier message, the apt. you liked best had warm furniture tones, but a very cool wall tone. I'm not sure what that will mean for your space but it does make me think you prefer a cool space and maybe you should just go towards that rather than trying to balance war/cool. Just a thought...
http://www.benjaminmoore.com/index.asp