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.