I have heard good things about Mayhems dyes. Also, their blitz cleaning kit works really well for cleaning rads. I have always used distilled water, if you want color without the problems of dies maybe look into colored tubing.
Mayhems has a good reputation. Even with distilled water I recommend using additive to stop the distilled water from pulling molecules from metal to balance it's own PH. I worked in automated car wash for years and we had to replace the stainless steel spray nozzles on distilled final rinse every couple of years because distilled water eat them into bigger and bigger nozzles.