Have thought a few times that being able to add a wash to foam flies or those constructed of a smooth material could be interesting. Currently been thinking about "dirtying up" some segmented foam flies by adding some sort of tint to thin head cement. Has anyone had a go at this?
get a cheap bottle of nail polish at the dollar store. comes in a wide range of colors or get some permanent markers. they too come in a wide range of colors
I love my bottle of 99 cent sparkle nail polish. Clear with lots of small flakes in it. When it drys and you rotate the fly you get little "sparks" of light jumping out at you.
If you're working with foam patterns, I agree with Norm, use waterproof markers that are available in a plethora of colors.