Bodhi's smart-arse comment aside, there are many references to it online. The yellow Graff myth has been perpetuated in print in several cases. All of them seem to stem from a single French pilot's comments about thinking he saw an all-yellow 190. The thinking is that he got it wrong, and that the bright yellow cowling streaking past at full speed could look like more yellow.
All accounts of the yellow 190 come from this, or from direct references to this pilot's single account. It's never been substantiated by any other sources as having existed. Graff was a well known ace, and if he had really flown a bright yellow 190, there would be tons of proof. He was quite willing to customize his ride (tulips, orange cowl, lightning bolts on the sides, fuslage art, etc) but never anywhere has it been mentioned he had a yellow 190.
EDIT: I skinned 2 of his planes in AH. I did a lot of checking on the matter, being curious about the yellow 190, some year ago or more.