Why do egg yolks change color from orange or dark yellow to yellow or light yellow when cooked, esp. hard boiled?
This is due to the proteins in the yolk denaturing and aggregating - basically the same reason the white (albumen) turns white from clear.
Think about the color that would result if you mixed a dark yellow color with some of the color white (i.e. I'm not talking about mixing yolk with albumen here, just colors) - it would lighten to make a lighter yellow