What food colouring can i use to make my pool blue?

You can make your pool blue by by putting blue dye in it, but make sure it is safe to consume and to have on the skin if people or animals are going to be in contact with it. Most pool users have the pool painted with water resistant paint and then when fully dried they fill the pool - dyes such as "NBS BIRU (Blue Dye)" and many other are available but the onus is on you to ensure the safety of teh water for what ever purpose you want to usei it for