* It: If you are referring to the lizard in a general way, "it" would be appropriate. For example, "The lizard was sunning itself on a rock. It was a beautiful green color."
* He/She/They: If you know the gender of the lizard, you could use "he," "she," or "they."
* The lizard: If you are specifically referring to the previously mentioned lizard, you could use "the lizard" again. For example, "The lizard was sunning itself on a rock. The lizard was a beautiful green color."
Example:
"The lizard was sunning itself on a rock. It was a beautiful green color. It flicked its tongue out and caught a fly."