Well People do use Anti-Freeze in their Watercooling setups to prevent corrosion (although there are better solutions).
If you want something just for looks, Green UV Dye is the best way to go. You can also use a highlight and dip it and squeeze out all the 'juice' into the water, it is also UV reactive and a cheap way to get good color.
If you don't care about UV reactive, food coloring is also good.
But both these wear out in about 3 months times.
-JokerF15