The coloration of a lizard has a lot to do with lizard behavior. If you look carefully around a lizard doing push-ups, you may see another lizard close by. During the mating season, males of most lizard species become brightly colored, generally in metallic blues, greens or yellows, sometimes in patches. Push-ups expose these patches to any lizard watching nearby. If the watching lizard is female, she determines the fitness of the male by the size and color of his patches and his ability to do push-ups and head bobs.
Lizards also do push-ups to regulate their internal body temperature.
Lizards are ectothermic, meaning their internal body temperature is close to the air temperature. A lizard doing push-ups may be trying to cool down by lifting its body above the hot surface. Additional behaviors regulating body temperature include basking in the sun to warm up, or hiding in shade, deep crevices or underground to cool off.