One, typically, has such questions until you actually see one up close (MONTAGE WH). Seeing it in person usually calms any fears you have about it. Once you see one, touch one, play one you stop worrying about such things... When you get the chance, go see one, up-close... you’ll immediately know you have no worries about it yellowing over time (or even showing dirt)!

As to your second question... that is a “Do these shoes fit my feet?” question... or better “Do these jeans make me look fat?” question. My only comment here is, the audience will, at least, think you SOUND good, how they think you look, is another thing.

The finish is unlikely to be much different than the white Motif XFs in terms of permanence. Those are much more "over time" tested than any Montage. And your eyes can tell you if you detect any yellow in any of them (the Motif XFs). I don't. Ebay, Reverb, Sweetwater, GC, etc (any place with used pictures) - they all look white to me.

Thanks guys, I really liked the white one from day one that I've seen it on the photos, so it will be the one. Maybe I'll see one in a store.

I was also looking for the Kronos, but as I've had the SY99, S90XS, Motif ES, Motif XF and I'm very convinced of Yamaha, I'll stay at Yamaha.
I own the MODX6 but was wondering if the Montage is worth the money. Therefore I compared the Montage vs. MODX in a store. I think yes, it is worth the money. The sound is fuller, keybed is very nice to play and I have 4 outputs, a better USB interface so I don't need an extra interface anymore, also when I use it live. Currently I use a RME Fireface UC where I route the click through output 3, I think this can be done via Montage then, too, right? I wouldn't need the extra interface then.

However, the MODX6 will stay as it is super compact and inside the hard case it still meets the requirement for standard luggage, which is good for flight gigs.