Why does light from a screen separate into three colours when you run your eyes across it

I've noticed a phenomenon whereby when you're looking at a modern television screen (plasma, I think) or a projected image, and you run (ie rotate) your eyes from left to right quickly across the image, it separates into the 3 primary colours of light. Why does this occur? Can you explain the physics behind it?

Thanks in advance

Most modern, inexpensive projectors just have a single, white light source with a color wheel in front of it with filters for each of the primary colors. The color wheel rotates really fast in front of the projected light. Each time a new filter passes in front of the light, that part of the picture (red, for instance) gets projected. When you run your eyes quickly, you are just seeing the downside of that type of projector.

Use a magnifying glass to see a closeup detail of the screen. You will see patches or stripes of three colors--red, blue and green. One set of three is a pixel. The variation of intensity among the three primary colors let many thousands of colors be presented in one pixel.