Matte only over items of a specific color?

Question

Example scenario:

I have 4 boxes. Box 1 is blue, Boxes 2 & 3 are a specific color of red and Box 4 is green. I would like Box 1 (blue) to only be visible when overlapping a red box. If Box 1 overlaps Box 4 (green) nothing happens.

My question: Is there a way I can matte a shape so that it is only visible when it overlaps a specific color? Essentially like a "Set Matte" effect that is driven by an RGB hex code rather than a layer's bounds.

Answer 1

Place a duplicate copy of your red box as the top layer. Put the blue box as the next layer. Set the Blue box's TrkMat (track matte) to Alpha.