When I got my snoot, it came with four discs of thin plastic gels in different colours. I got a red, blue, yellow and one of a grayish white colour, like milk with water. I found out, that for my purpose; (a ray of coloured light hitting the background), - that yellow looked really good, like a lime-green colour on a black background. The blue one was a pretty colour too, but the red was really not good at all, not on a black background. The white gel on the other hand, also made a really good effect, it surprised me.
On a white background the coloured light effect was different, the colour being much more subtle, like a soft halo when using the blue disc for example. The snoot used here, is set up close to the paper background, so the light drags a long tail down. By moving the snoot this effect can be very different. If the light is coming from the same angle as the camera, the light would have been a small coloured spot.
Also a snoot can be used as a hair light.