Guard against illegal write_graph() calls about graph-coloring

[brrcrites]

This issue is caused when the write_graph() function is called for this test case. The test case requires so many colors that graph_colors.h isn't large enough. We need to either guard against writing .dot files that require too many colors, or better would be to make a better write_graph() function that can create an infinite amount of colors without the need for a graph_colors.h

I am updating the issue to reflect the problem more specifically, and adding an enhancement to rewrite write_graph() without the need for graph_colors.h