I want my water to been green coloured because i want  to to look like a vat of radioactive waste.
Thanks

the surface you want to apply the water texture must be a terrain patch first
then "press g" i will open one window in which you have to selec color(you can use the slider) or colour palette by double clicking the color rectangle. then tick the color box.

after that select your patch and press v, select the verts and then using the window that opens when you press g select apply