As much as people have used all sorts of different dye's and such, NOTHING I have seen can compare with Dye Lite. (the original stuff I used in DH3.. you know the thing that started the whole "glowing coolant" deal on the web?
hehehe you can get it at most automotive places because its used for coolant leak detection. Its far more bright than any highlighter concentration I have seen, same with woolite.