This past weekend I experimented with dyeing some etched PCBs. I used Rit dye easily found in grocery stores. It was quick and dirty. I just boiled some water, mixed in a heavy concentration of black dye and let it sit for a couple of hours.
The dye worked well enough. Rather than just soaking in a cooling dye, I should have been applying a constant heat and agitating the solution in order to get a darker saturation. And when removing flux after soldering the board, dye came off as well. But in general this is a promising way to make nicer looking DIY PCB boards in the future.