[Dave at Pelican Parts]

There used to be an already-dyed brake fluid, ATE Super Blue. It is no longer available in the US.

Since most dyes you can get will be in water, it is a very bad idea to add them to your brake fluid. Others would be in the form of small particles, and I would be worried about those getting lodged in important places inside the system.

I wouldn't do it, myself.

--DD