An anaglyph is a stereogram which uses colour to seperate the two images for each eye. The image for one eye consists of red colour information, and the image for the other eye consists of green and blue colour information. There are more people with the right eye dominant than there are with the left, and there is more colour information in green and blue than in red, so usually the right eye gets the green/blue filter and the left eye gets the red one.
Through the red filter, green and blue are blocked, so black, green, blue, and cyan all look dark, and red, magenta, yellow, and white all look light (red). Through the green/blue filter, red is blocked, so black and red look dark, magenta looks blue, yellow looks green, white looks cyan, and all the other primary and secondary colours are pretty much unchanged. ("Monochromatic" anaglyphs would just use the colours black, red, cyan, and white.)
It is clear that some colour information is lost in producing a colour anaglyph, but the detrimental effect is surprisingly small - the brain makes up for the lack by automatically matching corresponding light and dark areas in each image, and by matching corresponding edges - so the stereoscopic illusion is not as bad as one might expect. With reasonable quality inks and filters, this is an effective and cheap way to produce static 3D images. 3D video anaglyphs used to be shown in movie theatres, and still occasionally crop up on television, in magazines and in comics.