No, you weren't thinking wrong.
There are two sets of primary colors: additive and subtractive.
The additive primaries are red, green and blue. However, the blue is not
what many people think of as "blue". They are used when dealing with light
oriented things such as photography. A proper mix of all three creates
white light.
The subtractive primaries are those you probably learned in grade school as
red, blue and yellow. However those really aren't the colors; the colors
are actually magenta, cyan and yellow. They are used more in applying color
*to* things as in painting and printing. A proper mix of the three yields
black.
Get the subtractive primaries and you are good to go. You might want to add
black as it is easier to create a shade (greyed down color) with it than by
adding the correct complementary primary.
More here...
http://en.wikipedia.org/wiki/Primary_colors
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dadiOH
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