I think to do this you would need to change the code for the DosPlotter object in pymatgen. Specifically line 140 in https://github.com/materialsproject/pymatgen/blob/5fb54181ddb90eb74f12bfe311be72b2b2fd0fd4/pymatgen/electronic_structure/plotter.py
140: colors = palettable.colorbrewer.qualitative.Set1_9.mpl_colors
If you would like to submit a pull request with the yellow replaced with another color, it would probably be well received.