This is additive color mixing of light. On the other hand, the mixing of filters rather than resultant light, works in a subtractive way. Mixing assorted filters in front of one beam will move its light towards black. White light is a mixture of all colors in the spectrum. By passing this white light through a filter, we are not coloring the light, but we are filtering out the other colors. White light can be split into three primary colors: red, blue and green. They can therefore be mixed together to make white light. A deep red filter will remove everything except the red part of the spectrum. Mix two primary colors and you will get a third color, mix other two of those primaries and you will get another color. Mix those two resultants and you once again get a different color. From here you´ll find information on how to mix Lee color filters to get white light.