The number used to define color in HTML is an hexadecimal number which is six characters long. The first two characters are for the red content, the second two for the green content, and the last two for the blue content. You can convert any RGB number to an hexidecimal number. These are the colors you should use, if you want to simplify your life.
Computer monitors use additive color.
Most paint programs allow you to enter RGB or CMYK numbers to create color. When dealing with HTML and mostly on-screen stuff you should always choose to work with RGB numbers. They also don't dither. Dithering is a process that reminds me of pointalism. The RGB numbers can range in value from 0 to 255.