You will find out shell lizard has a not-too-low probability to show up when breeding 4-color.
I can say that all of my 5 shell lizards are bred unexpectedly, because I always put 4 colors to breed.
This. I have three Shell Lizards and they all were from breeding four colors.
I play all the games but these days I have been showing more interest in Farm, Bakery, and Pet Hotel Story. But I do check the other games like three times a week. =)