Here's the basic formula for two color tint (you can also do it with non gray scale source image also, you can experiment with it.)
Darker colors are more tinted by dark tint color, lighter colors are more tinted by light tint color. It does take some playing with to find good combinations though.
texColor is the rgb color of the original texture. texColor.a is the alpha of the texture (used so we don't colorize transparent pixels.)
finalColor = (1.0 - texColor.rgb) * darkTint * texColor.a + texColor.rgb * lightTint;
The benefit is you can tint darker colors with one color and lighter ones with another color for more variety. It depends on your images though, if they are all one or two solid colors, then replace color might be the better way to go.
I added an example Two Tint effect to my C3 effects pack, so you can play with it, if you want. If you don't have a key of my pack DM me on Twitter and I'll send you one (kindeyegames on twitter.)