If you are referring to the color change...most darken the body color to warm up and lighten the color when they are too warm. They also use color changes to indicate mood and indicate whether they are gravid/non-receptive or not....but the color changes for mood and receptivity are also accompanied by posture changes.
For instance...a male veiled seeing another male veiled will inflate his body, raise a hand up near the chin, coil and uncoil the tail, turn bright colors/patterns, hiss, gape, etc. and attack the other male if he doesn't back off.
A non-receptive/gravid female veiled meeting a male veiled will hiss, gape, rock back and forth, inflate her body, display mustard/yellow splotches with bright blue dots and a dark, almost black background color.
A receptive female veiled meeting a male veiled will remain calm, body not inflated, no hissing, no gaping, hold her body close to the branch and move her tail out of the way and display the mustard/yellow splotches and blue dots....but keep the nice green background color.
IMHO they do not usually try to blend in to hide although I have seen some vague kind of blending with the odd species.
Hope this helps.