Depends on what subspecies of Jackson Chameleon you are getting. There are 3 subspecies available in the market; Mt. Meru or Dwarf Jackson's ( Chamaeleo Trioceros jacksonii merumontanus), Jacksonii Jacksonii ( Chamaeleo Trioceros jacksonii jacksonii) and Standard Jackson's Chameleon ( Chamaeleo Trioceros jacksonii xantholophus).
Mt. Meru or Dwarf Jackson's ( Chamaeleo Trioceros jacksonii merumontanus):
Smallest of all 3, females bears 1 horn
Male have yellow creast with bluish-green eye
Jacksonii Jacksonii ( Chamaeleo Trioceros jacksonii jacksonii)
female bears 3 horn just like male
Male is the most colorful of all 3 subspecies, with a yellow band run horizontally across the body, lots of blue and green
Standard Jackson's Chameleon ( Chamaeleo Trioceros jacksonii xantholophus)
female bears 3 tiny pin head size horn, also there were some "red-phase female"
Male, some blue-phase with yellow and green
Jackson were listed as one of the starter Chameleon; BUT personally I would go with the Standard Jackson to start with.
Here is a link to FLChams, Mike have all 3 subspecies and picture of them to compare:
http://www.flchams.com/other_chameleons.asp