This is yet another in a series of "grid" scripts. I tried to shoe-horn this one into the circular grid script (found here: http://registry.gimp.org/node/25206), but there were too many other variables that were specific to the diamonds. It also gave me a chance to do a bit of code clean-up.
As with the circle grid, you can do two colored diamonds, and each one can have their own width and height. The "outer" one is the one on the bottom, unless both dimensions for the outer are less than those for the inner, in which case the order of those two layers is switched. There are options for anti-aliasing the shapes (happy, mahvin? :-) as well as totally random colors. The attached image is the output of a default run of the code.
As always, let me know if there are any issues or comments on the code. Enjoy!