I think of E. coli being more white than yellow. When you compare E. coli and S. aureus on an agar plate, the Staph colonies have a much more distinctive yellow tint. The yellow tint in Staph colonies is due to staphyloxanthin, a carotenoid pigment.
You will be surprised that when describing the colonial morphology of E. coli on a Blood agar plate, the word choice for colour is "grey". I believe that user leonardo up there had a very good explanation about this "yellowish colour" question.