I am by no means an expert with modifications, however, I have used massive amounts of Apoxie sculpt in the few mods I have done. In my experience white Apoxie sculpt does turn a greyish hue after mixing while it is still mouldable. After it dries and you sand it, it does become a nicer white, but still not perfect. A lot of people I know that use Apoxie sculpt tend to lightly paint their puzzles to make it more pretty. Stickers will also cover up a lot of what you don't want people seeing.
I hope I helped a little, and good luck on your modification!