My male YB doesnt have alot of yellow on his belly either. He too is from a proven Ivory line. The father of mine has the most yellow on his belly ive ever seen. Like a solid yellow almost albino looking yellow. Then darker closer to the edge of its belly. But yet mine does not have that much. Not sure if it gets more yellow as it gets older. Mines only 300g. Here is a picture of my YB's belly. I wish I had a picture of the father, ill have to see if I can get one