Agreed that they need a small, warm basking spot (mid to upper 80's F) but that ambient temps in the 70's F are a good target. Please consider the important difference between ambient vs. 'basking' temperatures here. Likewise, in nature they normally see ambient temperatures (i.e., air temperatures or environmental temperatures in the shade) in the 70's F whereas an object exposed to the sun will easily reach much higher temperatures--lethal temperatures in fact, though a basking spot in the mid to upper 80's F should be plenty sufficient.
Young animals are darkly colored but they start getting adult coloration around 6 months or so. Assuming this is an older animal, IME dark coloration on Jackson's usually means either too cold (though you usually only see the dark coloration in response to light or a heat source, i.e., they assume normal green shades even if cool but aren't in bright light or don't have a basking spot), or unhappy about something, but we need more info to offer any suggestions. Pics of the animal and enclosure would be helpful too.
