I have limited number of colors but I do denim with blue and then with blue mixed with white
Leather, as has been told is dark brown with more khaki brown.
For zombie skin, I'd suggest you just get nuts and try things...theres no one skintone for decaying undead. I got them in normal caucasian, very pale, greyish, greenish, I've seen some purplish...everything works!
Oh, for paints, I use vallejo but I don't like them too much, as some paints appear to be old or something and are not mixed good. at some, a totally different color from the normal comes out first, at others, a waterish fluid comes out first...they have to be mixed I think, so it might be my problem. Also my vallejo wash has at first given only dirty water and now its too thick. But those are the only paints I use so I am really not to judge.