Mike has the right idea. I'll just add that if you cut the dark color into smaller chunks it will melt better. Also, add stabilizer [a few drops at least] to that fresh plastic and mix it in good before you pop the color chunks into it.
I make a lot of colors that are darker than what I want and have to thin them out a bit to make me happy.
On color density, the more mass a bait has the less light will pass thru the bait making the color darker. The same plastic/color in a small bait will be more transparent and lighter in color. You have to develop a feel for what colors will work best for specific baits. That said, let the fun begin.
