An actual preview system on owned wolves would be much more ideal- but for now, if the preview dummies could be changed to a shade that isn't white it might help with custom markings. My Foxify apparel adds white markings and during submission I was asked to rework and use a blank, solid color wolf for the icon- I actually had, but the mod was confused- I assume- because the actual preview they were seeing was already white, so my white marking additions were not noticeable at all and to them, it looked like I had placed my apparel on a wolf that had markings for the icon.
I think white will be common for these kinds of apparels, far more than a specific shade of grey, so perhaps that would be better. I also would like people to be able to see that my apparel does come with built in white- so they won't buy if they dislike this.