When considering pigments, blue, red and yellow are primary colors. These are called subtractive primaries because the base three cannot be changed, but they can be mixed together to get different colors.
When considering electronics, the primary colors are blue, red and green. These are called additive primaries because with the addition of more light, you can change the colors to anything you want.
Additives and Subtractives
Since pigmented primary colors are subtractive, the more colors you add to the mix, the darker the pigment becomes. Adding pure pigmented colors, in theory, will give you the color black.
It works exactly the opposite for additive colors. Taking the primary colors in this instance, in effect adding light even if it is a darker color, will ultimately give you the color white. As proof, look at any spectrum that divides light into it's basic colors. The light that enters the spectrum is pure white, then splits into every other color. Joining all of those pure colors in a reverse process naturally gives us back pure white light.
Knowing the basics of color reproduction with pigments, it is impossible to mix any pigment to make yellow. However, knowing that the light generated colors work in the opposite fashion, electronically mixed red and green will give us the color yellow. To make a lighter shade of yellow, take out the green and add in various shades of blue.
What Yellow Looks Like
A computer that generates colors senses colors completely differently than how people sense colors. Computers see numbers and letters to make calculations. In the computer calculation for additive colors, it recognizes "FFFF00" as the color yellow. However, the lightness of yellow are controlled by numbers. In simple terms, these start at the number 1 for a yellow so dark it looks black, to number 255 for a yellow so light that it looks white.