I love playing with water drops! Most of the different colours are achieved by reflecting my flash off coloured paper which tints the water (as such). Some colours have been highlighted in post-processing, but all have been achieved with:
a bowl; a flash; white/coloured paper; a zip-lock bag filled with water; a pin; a tripod, a camera; a fast lens; and alot of patience!
Click on the thumbnails to see a larger image size.