Nice looking calls. If you really want to get into dyeing wood check out the aniline dyes or the transtint dyes. They are made for wood and will allow the grain to show through better. Not sure on the cost versus the sharpies but my guess it would be less. You can also dip the calls in these dyes and use combinations of colors for different color effects. These dyes are the same as what some of the wood stabilizers use to color their wood.
Marvin