[GEM-dev] alpha blending of patterns

Hi I am having a slight problem and cant seem to find the remedy for it just yet...

What I am doing is creating an optical illusion system for an artist. The idea is that when the person is moving then they will appear in the pattern as a silhouette, but will have a pattern on them, and when their movement is minimal then they will disappear into the canvas again. It is a simple idea but there is one thing catching me out.

I am trying to map a pattern to just the colour black but alas am having no good results. Has anyone got an idea of how to just overlay on a single colour?

Thanks, Conor Higgins