Photoshop - You remember something better when you do it youself. Color models are RBG (Red, Blue & Green), CMYK (Cyan, Magenta, Yellow, Key line or thin black), HSB (Hue, Saturation, Brightness) and CIE lab.

The RBG model, which computer monitors and TV screens use for display, assigns values on a scale of 0 to 255 for each of the three RBG primaries.

CMYK model is used to print high quality images. Normal printers can deal RBG models too. Ink jet printers use cyan, magenta, yellow and black inks for printing. When dealing with light, colors are additive, if you are using paint or ink on a surface, colors are substractive. That is when we mix RBG light, we get white light. If we mix RBG paint, it will be muddy brown.

To change mode from RBG to CMYK, Image->Mode->CMYK.

To convert a color photo to grayscale (256 shades of gray), Image->Mode->Grayscale. Warning : color information will be discarded.

RBG is the preferred color mode for working on pictures. Web designers are limited to 216 (web safe) colors. Of the 256 colors, 216 are identical for Macintosh and PCs. Using indexed color, more colors can be used in the web.

Image->Mode->Indexed color. Palette : web.