eyes alert and not sunk in. Tail looks muscular and not thin. Active not closing its eyes during the day. No sign of mouth rot or wounds to the body. Sitting normally without pointing nose up towards the lights.
I look for health over color... The examples above indicate health or issues. Depending on where you buy one from you are either at high risk for health issues or very low risk.
Seriously, if you get "the good one", its not really the good one. First thing i ask is "can i see mom and dad", then its off to the food section and the rearing cages.