Hi tony,
The amount of food eaten per day would depend on the size and type of food. Some people might offer contrasting advice, but for young chameleons like you have I usually feed them as much as they will eat once a day. This usually ends up being from 10-20 food items. Make sure not to feed insects that are too large.
Chameleons do not change to their surroundings, although natural selection sometimes makes it look that way. Some things that do cause color change:
light intensity
temperature
emotional and physiological responses
In general, a dark coloring (brown/black) shows your chameleon is stressed from one or more of the categories above.
Actually, I need to edit the general statement about (brown/black) coloration. This is also dependent on the particular species, sex, and age of the chameleon.