Thomas Young, in the early 1800s, showed that a broad range of colors can be generated
by mixing three beams of light, provided their frequencies were widely separately.
When three such beams combine to produce white light,
there are called primary colors.
There is no single unique set of these primaries, nor do they have to be monochromatic.
The three components (emitted by three phosphors) that generate the whole gamut of hues
seen on a color TV set are Red, Green, Blue.
Looking through a colored window or cloth is another story.
Yellow cloth, paper, dye, paint, and ink all selectively absorb blue
and reflect what remains - yellow - and that is why they appear yellow.
This java applet let you play with mixing light beams and paint pigments.