GratingStim with two non-opposite colors

Hi! I am creating a following image in the Coder using visual.GratingStim:


I am wondering what is the simplest way to define both colors of the square grating, that are not opposed to each other, or at least to cover the non-white bars in the grating. In this case, I would have white and background gray color bars. I tried with GratingStim.color, .setBackColor, etc., but the second color still gets automatically chosen and I end up simply with a black and white grating.