You will get approximately the average of the two numbers you use. So you're better off to use a 15 and an 8 if you want a darker head kid.
15+8= 23, then 23/2= 11.5, so you will probably get a 10, 11, 12 or 13.
But if you use 23+8= 31, then 31/2= 15.5, so you'll probably end up with 14, 15, 16, or 17.
Sometimes it takes a few generations to get all the way to a varied tribe again. Just keep making matches until you're there!