Why is it that televisions and computers use the colors red, green and blue to display colors when the three primary colors are red, yellow and blue?
Because red, green and blue are the additive primary colors, and televisions and computer screens perform additive color mixing. Mixing paints and dyes is a subtractive process, and technically, the subtractive primary colors are cyan, yellow and magenta. This site has some background:
It includes a discussion of the Red / Blue / Yellow primary set as well as additive vs. subtractive mixing.
And I wonder: why is a site about color mixing done in such an awful mish-mash of font and background colors?