Most people use a phosphorus-free calcium powder at most feedings...a phos.-free calcium/D3 powder twice a month and a vitamin powder containing a beta carotene source of vitamin A twice a month since beta carotene won't build up in the system. If you need to provide prEformed vitamin A, it can be given using a different vitamin powder less frequently so that you don't overdose the chameleon.
Calcium, D3, vitamin A and phos. are all important players in bone health and need to be in balance.
Gutloading, UVB light and temperature are all important in the health of the chameleon too.