Suppose per vertex colors are passed to the vertex shader. A triangle is sent down the pipeline with vertex colors $(1,0,0,1),(0,1,0,1),(0,0,1,1)$ for p$1,$ p$2,$ p$3,$ respectively. At the point p $=$ up$1+$vp$2+$wp$3,$ the color of a fragment at the point with barycentric coordinates $(1/2,0,1/2)$ is $(1,0,1,1).$