Hi Pedro, I either use a coloured bowl with my water in or use a coloured backdrop, or even a multicoloured sheet under a glass bowl. I've also used scrunched up baking foil too and a torch. Also coloured lighting helps. I mostly experiment but you can also add colour and tints in Photoshot afterwards. There are quite a few tutorials on here too so I will point you in the right direction once I find them. Hope this helps a little.