skeleptica....Different species of chameleon have different colors and patterns than other species. Some chameleons (like panthers) have a dramatic range of color changes while other species (like Jackson's) can only make simpler changes in color and pattern.
Chameleons change color for several reasons. Postures will differ too.
They change colors and body shape to control temperature. A chameleon that is cold will flatten its body and angle it towards the sun and darken the colors so that it can warm up quicker. If it gets too warm, it will make its body size slimmer and turn to a lighter color and maybe even gape to cool off.
Chameleons also change colors and patterns and posture as a form of communication. Each species has different patterns, colors and ranges of colors that it can change to. I'll use the veiled as an example...if a male veiled meets another male, it will flatten its body, brighten its colors, hold one hand up close to its body, extend the gular area, curl and uncurl its tail and sway/lunge back and forth to try to scare off the other male. If a male veiled meets a female veiled he will have similar coloration and flattened body but won't hold the hand up. If a female veiled is not receptive or is gravid, and she meets another male, she will turn the background color almost black, gape, hiss, sway/lunge and even bite if the male comes close enough in order to get the message across to him that she is not interested in mating. A receptive female veiled will have bright torquoise blue spots and almost hug the branch and slowly walk away from the male and let him approach and mate with her.
Here's a gravid/non-receptive female veiled...
http://pets.webshots.com/photo/2156684130058457964rJlAJz
http://www.wildeyereptiles.com/cpg132/displayimage.php?album=34&pos=3
Normal veiled female coloration...
http://pets.webshots.com/photo/1174628161058457964mAcJpI
Male veiled courting the female...
http://pets.webshots.com/photo/1147641220056094951bEUlFs
Veiled sunning but the background color isn't as dark as it could be if the chameleon was quite cold...
http://pets.webshots.com/photo/1147678562056094951XZUxnl
Veiled, somewhat defensive/aggressive...
http://pets.webshots.com/photo/1147686183056094951sfHWcZ
Gravid/non-receptive panther chameleon...receptive colors are paler by far...
http://adcham.com/images/images-species/F.pardalisAmbanja/ambfemaleoneal.jpg
Panther chameleons from various areas/morphs...
http://adcham.com/html/taxonomy/pardalismorphs.html
Hope this helps!