If it’s a linear gradient from edge to edge, or even diagonally from e.g. northeast to southwest, then you don’t need to worry. You assign colours to your four vertices, as in the example in my first post, and it blends between them as you would expect.
It’s only a problem if the four corners do not correspond to a linear blend. Daniel’s example has reddish colours at opposite corners and blue-green at the others. That is not a linear blend, so it looks off.
To add a vertex you need to make a mesh yourself; one with five vertices and four triangles.