Recently they changed their rules, could be 10% non-GW material. Now it has to be 100% GW or hand crafted. Greenstuff sculpting, or you have to prove you DESIGNED and 3D printed any bits, printing someone else's files gets you the boot.
On contrasts, I've tried 3, Apothecary White, Black Templar, and Ork Flesh. Black Templar works fine, nothing to write home about, White is essentially a wash for something already white, glad to have it for white hair/fur, but it makes what your painting less white and I could never do detail with with it. Ork Flesh was probably the first I used and I might of used it improperly, but I've put it to some good use giving edges to necron phase blades. Considering the yellow.
In general, my experience with contrasts is that they are great washes over real paint.