Its normal. My focus s had really bright whites for the buttons. Nokia seems to like the off white, and I do too, it goes with my black 920, gives it a professional look.
Yep, it's normal. I asked this same question when I switched from an HTC Trophy to my Lumia 900. It is noticeable at first, but now I prefer it to the bright white.
My red one has bright white, my wife's black one has the slight yellow you speak of. My cyan 900 had the slight yellow as well, I wonder if it is a matte color vs glossy color thing?
No this is not normal. This is defintely a defect. I just received my second black lumia 920. The first one had a broken ear piece. This one came with the yellowish greenish buttons you speak of, unlike the first black lumia 920 which had bright white buttons. Real dissapointing. Hope #3 will have a working earpiece and the white buttons.