This rather large specimen has yet to reach maturity; when he will change color to a bright yellow and beep blue rings on its tail and body. There is another reason it is this color: An iguana who is under severe stress will slowly turn from green to dark brown, then black. The color change will start first on its body and head, extending down its tail, legs and belly. The belly and sides may first go from green to yellow before finally going brown and black.
This type of color change is often due to psychosocial stress, such as when an iguana is housed with another iguana or more than one iguana who is intimidating it. The iguana whose color changes is the subordinate iguana and, if you watch carefully, you will see that it is being kept out of the basking areas, away from food (or from getting enough food) and is often to be seen at the bottom of a pile of basking iguanas.
0 
