White light consists of frequencies of all colors: violet, blue, green, yellow, organge and red--blue, green and red are the primary colors.

A red filter only lets the red photons pass through, it reflects and absorbs photons of other colors.

Put a green filter on top of a red filter, what color will you see on the downstream side?

The green photons pass throug the green filter but cannot pass through the red filter. If both filters are perfect filters, you see perfect black. If the filters are less than perfect, you see darkness.

If you mix paints of blue, green and red, what color will you see? It will be a dark color.