Off-center color shift is an issue with all focal lengths on the M9, even 90mm and this is why Leica programs in settings for Leica lenses into the M9. With Leica lenses, or ones that are reasonably similar to Leica lenses (eg the lovely and value-priced Zeiss ZM 35/2 Biogon), you’re in reasonably good shape— you can manually choose the specific lens (for non-coded or non-Leica lenses) or set it to Auto for 6-bit coded Leica lenses. But even that is not perfect, because even Leica lenses vary from sample to sample.
The wonderful variety of lenses in M-mount (or 39mm screw mount with M adapter) means that this issue is just not going away. I feel that Leica has a lot of homework to≠ do here to carry on the M tradition: non-Leica lenses deserve better support than just “closest Leica equivalent”, which doesn’t work well at all for some lenses.
For example, I have some Voigtlander M-compatible lenses coming courtesy of Stephen Gandy at CameraQuest. I will be shooting them this weekend for notes in my M9 review. With about 13 Voigtlander lenses and the same number of Zeiss ZM lenses and many others, Leica M users deserve more extensive lens support in their US$7K M9. It’s fair that Leica can’t do this all at once, but a camera system deserves that sort of commitment.
All that said, I’ve developed as an experiment a layer mask for Photoshop which can largely eliminate the color cast issue even with the 21/4.5 Biogon. Not perfect, because the shift varies slightly with the lighting and aperture, but really much better than without. Black and white shooters can pretty much ignore the whole affair. I also have the Zeiss ZM 18/4 and 21/2.8 which I’ll be evaluating.
You can read another person’s analysis here, though it’s way too nerdy for even me to want to wade through.