There are 3 dimensions to the full color spectrum: red, green, blue. The most accurate way to portray this spectrum is a 3D cube. This page is 2D, and as such only 2 of the colors may be fully examined at a time, think of it as a cross-section of the cube. Each color has 256 possible values (0-255), and with three colors that gives a possibility of 16.8 million colors (256^3). When you choose a color above, the 2 colors that you did not pick are placed on the x and y axis. It is easiest to imagine that the slider now becomes the z-axis of the chosen color, use it to determine which cross-section of the 3d color spectrum cube you want to look at.