Actually I have experiences of the Valks yellowing in different degrees, but the truth is that all plastic yellows, even clear ones, just how obvious, and how much. In the best case scenario, I had the initial release of the 1/48 Hikaru VF-1A, that after some 8 years, has yellowed significantly, though evenly. In my worst case, the initial release of the 1/60 V1 Hikaru 1A (Again?!) has its yellowing very unevenly. the landing gear doors has a way yellow-er hue than most parts, for example. Probably the composition of the different batches of ABS plastics.
White has to be the most unforgiving of all colors as obvious yellowing goes. It is just too obvious. I don't need a comparision to a newer specimen to see that it has yellowed. It is detectable just like that.
Are you sure the SNES cartridges have not yellowed? You can't really be sure until you have compared it a very recently produced cartridge. Only that way you can be sure if it has yellowed. I have a Low-vis 1/48, and while I don't detect any yellowing right now, I'm sure if I were to compare to one that has has been stored in the correct conditions Mint in the box, i 'm sure there will some some difference that I can make out. Of course you might say that the newer/ differnt batches of plastic might have a different hue, but when certain parts of the same toy/ object/ whatever starts to exhibit slightly different hues of color (like my 1/60 VF-1A), you know that is happening.
In anycase, this serves to substantiate that darker colors renders the effects of yellowing less obvious.