Change Color in jit.gl.gridshape
the gridshape's color attribute is used to change the color, and in your patch it is working as expected (throw a jit.pwindow after the capturing gl.node to see in action). Your problem is in the feedback jit.gl.pix process, where there is some confusing logic that's is swapping color planes and doing some math. chopping that logic gets your color back in the feedback output.
Max Patch
Copy patch and select New From Clipboard in Max.