The sunny day background is the most common, most of the turtles prefer to come out in the daytime. That is not to say a day turtle is common, other features may be rare or even one-of-a-kind.
The moonlit nighttime background is rarer, there are fewer nocturnal turtles and they only get spotted at night.
The rarer the turtle, the higher it's value. A turtle becomes rare when one or more of it's accessories or body parts is rare. Some accessories & body parts can only be found a few times and some only once!
Mythic tokens are turtles of legends and lore, only a handful exist. Out of the 20 that could have generated, only 9 did. These tokens have unique backgrounds and all rare parts. They are the rarest of all the turtles and fetch the highest price.