This is a demonstration of RGBA color for the Web. Imagine three spotlights, with dimmer controls calibrated from 0 (off) to 255 (maximum brighness.) The three lights are set to converge in the middle. By sliding the dimmers up towards maximum, the colors can be mixed, changing as the light gets brighter. Computers store colors as numbers, in Base 16 notation (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, 10). The numbers correspond to the brightness of the Red, Green and Blue phosphor dots or LEDs that make up the monitor screen.
An additional dimension of control was added with CSS3: Opacity, or the Alpha channel. In digital media, an alpha channel is a mask that designates some level of opacity for all or a portion of the image. In RGBA color, the alpha channel indicates the opacity of the colored element. The lower the number, the less opaque and more transparent it is, allowing whatever is under it to show through. The number can be represented as a decimal fraction from 0.0 to 1.0, or in percentage from 0 to 100%.
The ‘A’ slider represents opacity. Pulling the slider down reduces the opacity, showing the corresponding opacity numbers above the slider. Granted, the spotlight metaphor breaks down here, because light is transparent, not opaque; however, this does demonstrate the issue of opacity in CSS3. ☺