You probably learned in elementary school that there are three primary colors: yellow, red, blue. What they didn't tell you was that these colors are only the primary colors for a particular "color model" called the RYB model(after the colors). This model is further classified as subtractive, meaning that the color that you see is whatever is not absorbed (and thus reflected). For example, yellow looks yellow because red and blue are removed from the light.
It actually turns out that this model isn't the optimal subtractive color model out there, the best is called CMY[K] after the colors cyan, magenta, yellow, (and black). If you noticed, more recent printers have started to use CMY as their color cartridges over RYB for this reason.
On the other end of the spectrum, there are additive color models, where the light you see is a pure combination of colors, no fancy reflection tomfoolery here. There are also primary colors for these sets of models, with the most common being red, green, and blue, known together as RGB. Additive color models are the much more physically interesting ones, since it is easier to create objects that emit light of a certain color on demand, but fairly difficult to create objects that can be controlled to subtract light on demand. As a result, the computer world uses the additive RGB color model to produce colors. In many applications there is a fourth value that is associated with this model called alpha (abbreviated as A), which controls how not see through an object is. No Alpha, can see right through it, Max alpha, you cannot see through it.
Since additive colors were probably never taught to you in elementary school, I'll give a quick overview. As it turns out the three secondary colors of the additive RGB model are YCM (the primary colors of the subtractive model CMY) in that order. For example, Max red + Max green = yellow. If you want crazier colors than these then your best bet in the future is just to google "COLORNAME rgb", or if you want a list of colors just search "rgb colors". My first result is here.
Pretty much every application that asks for a RGB[A] color expects individual values ranging from 0-255. Sometimes you will see the value in hexadecimal which range from 00-FF and are prefixed by a #.