Color filters may be used in combination to achieve special colors that are not available as a standard catalog item. The illustration shows what would happen if you were to use a primary red and blue filter in front of a 'white light'. (fig. 1). The red filter blocks the blue and green, everything except the red wave lengths. The blue filter blocks everything except the blue. The result is a blackout.

Create a unique color by combining two color filters (fig. 2). Using an orange filter which allows red and green to pass in combination with a violet filter which allows red to pass will give you a new color in the red family. Combining color filters is a way to create colors not available in your swatchbook.