When you look at green dots with a red filter what color did you see?

-- Looking at the dots casually, they look green because they absorb all other
colors of light, and only green light is left to proceed to your eyes. (In order for
this to work, there has to be some green in the light shining on the dots.
Daylight and most light bulbs work fine.)

-- The filter looks red because it absorbs all other colors of light, and only
the red light is left to pass through the filter and come out on the other side.

-- When the green light from the dots hits the red filter, it's absorbed in the
filter, and there's no light left to come out on the other side.

If you're looking through the filter at the dots, they look black.