Solid

Solid is a very simple Matchbox shader that outputs a solid color.

Yup. That's it.

Why? It's useful inside Image effects to use with blending modes to achieve interesting color tinting or other effects. For instance, create a new selective, attach id_Solid to it, and pick a color. Then change the selective's Pipeline mode to Comp, switch the blend mode to something like Overlay and adjust the transparency to taste.

Grab a copy of Solid from the Logik Matchbook.