Additive vs. subtractive color mixing

[Aquarius]

I'm hoping that some of you more technically inclined people can help me with something that's mildly bugged me for a while, but now that I encountered it in my computer class today and was not given a satisfactory answer from my instructor, maybe someone here can help me make sense of this.

We started to talk about RGB color, how for a video monitor colors are separated by red, green, blue, right?

So I posed the question, what about yellow? Because the color wheel *I* learned in high school art class and beauty school uses three primary colors--red, blue, and YELLOW--and from there you can mix them to make secondary colors (green, orange, violet), and from there you can mix a secondary color with a primary color to make tertiary colors.

So teacher says I'm right so far.

SO...I don't understand how you can start out with two primary colors and one secondary color and end up with the whole spectrum when you mix them. Where does the yellow go? Or maybe more importantly (or more accurately), where the hell does it come from when you see it on the screen, if it's not one of the colors you're starting with???

The instructor said that this is because when you're dealing with pigments like I do, it's "additive color", as in you're adding and mixing colors to get a desired result. With light/video, it's "subtractive" color.

Riiiiiiiiiiiiiiiiiight.....

So, once agian, as far as I know, you still need some combination of blue and yellow to get green, so how the crap do you GET yellow if that primary color isn't being used????

So confused....

[enterprikayak]

Ditto that dude. I've always wondered that, and always I have been too lazy to Google it.

[Escriba]

As I get it, "additive colors" are, in other words, light. They're light sources: blue, red and green, that combined create the other colors. While "substractive colors" deal with the electromagnetic spectrum, witch parts of the spectrum aren't absorbed by them.

To make an example --that you can find in wikipedia-- when an apple is seen under a white light, it's red, which doesn't mean it emits red light (this one would turn it into additive color.) What the apple does is absorbing some of the wavelenght of the white light, only reflecting the ones that Humans see as red.

From wikipedia too:

Subtractive color systems start with light, presumably white light. Colored inks, paints, or filters between the viewer and the light source or reflective surface subtract wavelengths from the light, giving it color. If the incident light is other than white, our visual mechanisms are able to compensate well, but not perfectly, often giving a flawed impression of the "true" color of the surface.

Conversely, additive color systems start without light (black). Light sources of various wavelengths combine to make a color. In either type of system, three primary colors are combined to stimulate humans’ trichromatic color vision, sensed by the three types of cone cells in the eye, giving an apparently full range.

According to this, you can create yellow with lights, but not with, you know, crayons.

I don't know if I've made it any clear

[WarpGirl]

OK what I know about computers can't even fill a thimble, but I did take color theory when I was an Art History major. And everything Escriba said is perfect. Now what that means in computer world????? I'll ask my brother.