Masking a tile pattern for the Tile Sampler Color node.

I've got some clumps of grass that I'm putting into some Tile Samplers to make distributions of them. Grayscale clumps go into a Tile Sampler, and color versions go into a Tile Sampler Color with the same settings. The problem is that the color patterns stamp on top of each other as square tiles, while the grayscale clumps act as if they have alpha information (or mix together as if they're all an Add blend). A couple images are attached to show what I mean.

I'm looking for a way to mask individual pattern inputs for the Tile Sampler Color. Anyone have any ideas?