I don't have a DSi, but I expect the problem will be as prevalent as it is with the DS Lites. I've seen more DS Lites than I care to think about, and it seems the Black (Onyx in the US) variants suffer the most from 'yellow' tinted bottom screens. As some people have pointed out, if you were to compare two LCD panels both displaying 'white' screens side by side, it is likely there would be a slight difference in hue between the two so it may be unreasonable to expect the DSi screens to match. The annoying thing is that not all DS Lites and DSi's are affected.
My duaghter has a turquoise DS Lite (imported a year ago) and that has perfect screens, I have an Onyx version bought in the US and the bottom screen on that is very yellow. I have a theory that getting a 'perfect' DSi or Lite is all to do with batch numbers. A friend of mine recently purchased a DS Lite (Black) Brain Training pack from Amazon - it's perfect, even the volume slider is nice and tight, as the Amazon deal was good I've bought one, again, it's perfect - screens match and the volume slider doesn't feel 'loose'. Both serials start UEH, I can't remember the last time I saw a black DS Lite with matching white screens, let alone two.
It must also be rembered that if you have a black DSi or Lite, any colour differences between the two screens will possibly show more than on lighter colour versions. That's because if you have a black background or border to a colour, there is an optical illusion that the colour is more intense. If you take two sheets of card, one black and one white and cut an equal size hole in the middle of each and then put each of the sheets of card over another card coloured red, then, if you were to look at the two side by side, the red when viewed through the black card would look more 'intense'.