I bought three squirt bottles at our local dollar store (I love dollar stores!). I planned on filling them up with the primary colours of red, yellow, and blue, and then we could make other colours on the snow as we played. But - the kids didn't want yellow. They wanted green. I tried to convince them, but they weren't interested, so yellow was out, and green was in.
Come to think of it - colouring the snow yellow is a little distasteful!
To make the coloured squirters, I just filled them with water and added food colouring drop by drop until I thought it was dark enough to contrast easily on the white snow.
Just a warning here - the food colouring can stain, so make sure that if they do get some on their winter clothes you wash it right away!