What to clean Krenit bowl with?  

Probably not the 'right' thing but...
I used a product called 'Cerama Bryte' (glass cooktop cleaner) which seemed to do a really good job but I later read that that per the manufacturer it is bad for enamel/porcelain as it may dull the finish.

Thanks. I will give olive...
Thanks. I will give olive oil a go and see what happens.

?
Would toothpaste be too abrasive? Then a thin coating of extra virgin truffle oil? Or, Bar Keeper's Friend? Or consider leaving it alone. It took the piece several years to "look like it's looking". You will be removing the lovely bowl's sordid history.
But then again, I'm slightly buzzed.
Aunt Mark

I used olive oil and it...
I used olive oil and it worked wonders. Really blackened the outer steel but you still get that worn look shining through. Only time will tell if there are any qualities in the oil that will have ill effect. I will be sure to post my findings here 🙂

.
I want a Krenit bowl. I've never owned one. Were they manufactured previously by Knoll? Is there a desired color? Christmas is coming.
With affection,
Aun't Mark

I now know more, thanks to Miss Google.
I'm diggin' the large shallow bowl in the citronella colored green. I'll proudly post it when it arrives in the appropriate thread.
Eskimo kisses from Florida,
Aun't Mark

They
are being reproduced now- with some new colours!

The reason oil
-- any roughly transparent oil -- would make colors brighter and dark areas darker is because it -- like any liquid -- saturates the surfaces and changes their reflectivity. A polished stone is always darker and richer than in its unpolished state -- and oil, wax, water or spit will all, to some degree, replicate "on the cheap" the effect of polishing those surfaces.
Personally, I wouldn't gunk up a favored object with a (temporary) surface treatment which it would otherwise never have suffered. A good washing with some liquid detergent would be the remedy -- and the original treatment choice, as well. Any time you want to artificially enhance the colors, you can just spit on it !

I tried oiling a Krenit bowl
that was badly faded and it looked better for a little while but then it went right back to looking faded.

.
I plan to spit on my future Krenit...after a smooth highball., and a puff.
Aunt Mark