The 4 planes of the matrix in the jit.matrix object are alpha, red, green, blue (char data). The planes are numbered starting from 0: alpha=0, red=1, green=2, blue=3. The argument '3' of the jit.fill object specifies the value of its 'plane' attribute. Change that and you'll change the color.
You can read about this stuff in:
"What is a matrix?" http://www.cycling74.com/docs/max5/tutorials/jit-tut/jitterwhatisamatrix.html