In a nutshell: the eye has 3 different types of color receptors that are sensitive to red, green and blue light (it also has a 4th type which is much more light sensitive but doesn't allow for color vision - this type comes into play when it's dark). If all 3 are stimulated equally, you see white. If only one is stimulated, you see the corresponding color. In the end, all colors you can see are created by stimulating them to various degrees.
Substances have colors because they don't reflect all colors equally. A red one reflects only red and absorbs green and blue. A yellow one reflects green and red and absorbs blue.
In transparent substances, there is no full reflection but it is the absorption anyway that is the key property. Again: it's red because it absorbs green and blue. A substance that absorbs only green would look magenta, which is a bluish red.
So red goggles protect against green (but not red!) light because they absorb - swallow - green light. It's that simple. Complexity is introduced in the case of laser goggles because a) you should block only a small region around the wavelength in question, and not others, to result in high visual transmission, i.e. the glasses shouldn't become so dark that your view of other things is impaired; and b) the dye shouldn't degrade over time due to bleaching by ambient light or chemical instability (not a problem with synthetic dyes) and it also shouldn't (quickly) decompose in the intense light=heat of a direct laser hit.
There is also the possibility to create colors by interference (like in the proverbial butterfly wings) and so you can buy interference filters for various purposes as well. As mentioned, these can be very narrow bandwidth and are always pretty expensive. One can easily recognise them because light that's reflected off them has a different color than light transmitted through them.
So you are saying that most of the non certifed "Hobby Grade" glasses are dyed plastic?
Actually I'm convinced that all laser goggles, whether certified or not, hobby or pro grade, that you can get for 3-digit $$ figures are dyed plastic.