Robert Citek writes:
> I'm slowing getting familiar with gimp/gimpshop.  And so I thought I  
> would try a couple of toy problems.  One toy problem is to create a  
> venn diagram.  The resulting image would look something like a Visa  
> logo, except the colors of the circles would be blue and yellow, with  
> the intersection being green. 

Using layer modes, you can get an effect similar to what you're
describing: you can make overlapping areas of layers turn
colors that reflect the addition or subtraction of the two colors.
Unfortunately, blue and yellow don't combine to make green in either
addition or subtraction mode; they make white. Think about the RGB
values of the colors to understand why -- if you don't know the RGB
values of colors off the top of your head, watch the sliders in the
color chooser dialog when you select the colors to see how red,
green and blue combine to make each color. Blue is 00F, yellow is
FF0, and adding them makes white, FFF, instead of green, 0F0.

To get a better feel for how colors combine, try this exercise:
make a black background layer. On top of it, make three circles,
each in its own layer, one colored red, one green, and one blue.
Move the circles so that they overlap each other partially but not
completely. Now, in the Layers dialog, set each of the three circles
to Addition mode and watch how they combine. Play with circles of
different colors in different layer modes to see what happens.

> To clarify, I'm not looking to select the intersection based on color  
> and then fill the selection with green, but rather have gimp/gimpshop  
> imitate what one would do in the real world with color filters, e.g.  
> acetate[3], and a white light.

Subtract mode does basically what colored filters would do to a
white light (do the circles exercise I described, but start with a
white background instead of black). Addition mode is what you would
see if you shone lights of different colors (e.g. a blue light and
a yellow light) onto the same surface. Unfortunately, in neither
mode will blue and yellow combine to make green, even though that
is the combination you'd expect if you're used to mixing paints.

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