Most 2D graphics programs are able to do linear gradients with arbitrary orientations. If you don't mind a little work, it is possible to set this up to imitate the 2D linear interpolation across a triangle.
Set up two layers:
Pick two of the colors, say red and green, and set up a linear gradient between those two colors along the red-green edge.
In a ...
A vertex consists of not just the position, but all of the values passed from the vertex processing stage to the rasterizer. So if a vertex gets clipped, the new vertices generated in that process must generate new values for those vertex processing outputs. It does this using the same math that it uses to generate the new vertex position.