Hi Mamaison,
I spent a number of years in theatrical lighting, as a student, a
technician, and a designer. I always loved both learning and teaching
this particular subject!
First, some background...
There are significant differences between color theory in light and
color theory in pigment (like paint or dye).
When dealing with light, white is the presence of all colors, as
opposed to pigment where white is the absence of all colors.
The primary colors of light are red, blue and green (red + green =
yellow), as opposed to pigment where the primary colors are red, blue
and yellow.
Pigment is additive. If you want purple, you take blue pigment and you
add red pigment to it. Light is subtractive. To get purple (magenta)
light, you start with white light and pass it through a filter that
does not allow green light to pass through, thus subtracting the
green.
The gels used for stage lighting are filters. When you look at a
magenta gel, you are looking at a piece of glass or plastic that
reflects green light, rather than allowing it to pass through.
Similarly, when you look at a blue shirt, in a sense, you are looking
at a shirt that is every color except blue. The fabric absorbs red and
green light, but reflects the blue light. Your eye sees reflected
light, so the shirt looks blue. A truly black object absorbs all the
light, a truly white object absorbs none. As a side note, it is
unlikely that you have ever owned a man-made object that is truly
black. Black pigment is usually actually an extremely dark green or
blue or brown. This becomes very obvious when your black clothes start
to fade and take on a greenish tint.
When white light strikes a colored object, the object absorbs a
portion of the white light. The portion that it does not absorb is
what you percieve as the color of the object.
This is something that theatrical lighting designers must be very
aware of. If the costumers have dressed everyone in green and the set
is painted green, then using magenta gels will make the whole thing
turn an ugly muddy grey-brown. If everyone has somehow managed to use
dead-on primaries (unlikely) it will all look black!
There is an excellent explanation of this theory with good examples
here:
The Physics Classroom
Lesson 2: Color and Vision
http://www.physicsclassroom.com/Class/light/U12L2e.html
This subject can get much more complicated, in terms of the reflective
qualities of the object, the "temperature" of the light source, the
effects of diffusion, etc. Based on the category and phrasing of your
question, you seemed to be requesting a practical, rather than a
scientific, understanding of this, so I stuck to basic practical
application. Please request clarification if you need me to provide a
different type of information.
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