As teh esteemed Mr. Lee says, the base color used (or blocking coat) will make a BIG difference on the final color of the more transparent dope colors. Yellow being th eworst, and followed closely by red!
When in a discussion one day with Billy Werwage, he told me he uses a VERY light gray blocking coat. It covers (it doesn't have to be 100% opaque!) and helps give true colors. Windy use to sell pigment in white and yellow to add to dope to make it more opaque.
Bill <><