This is a bit of a summary from BiS's video:
All creatures can use all 12 standard skin colours and 7 pattern colours. However, the "true" combination for each animal (the one seen in the icon) will be the default (which may require unlocking in Challenge Mode).
Legacy skins (and therefore model variants) from RtJP cannot be given a pattern (and it seems like there's only one of each, no more ABCDE skins). Additionally, some parts of the animal's pattern are unchangeable (ex. the coloured head of Edmontosaurus), and are directly tied to the skin.
8 standard skins and all 6 applied pattern colours must be unlocked from the Campaign Mode. BiS didn't mention how the Death Valley, Salar Del Huasco, Yukon River, and Mangrove Forest skins are unlocked. It's possible that these are tied to the unspecified unlocked skins from beating Challenge Mode, or simply unlocked by default.
Standard Skins (12):
- Sonoran Desert - Desert
- Death Valley
- Great Sandy Desert - Desert
- Champlain Valley - Temperate
- Salar Del Huasco
- Limpo River - Temperate
- Qilan Mountains - Taiga
- Yukon River
- Svalbard - Taiga
- Amazon Rainforest - Tropical
- Mangrove Forest
- Gambia River Basin - Tropical
Patterns (7):
- Rana - Unlock 1
- Chalcorana - Unlock 1
- Lithobates - Unlock 2
- Pulchrana - Unlock 2
- Papurana - Unlock 3
- Pelophylax - Unlock 3
- Blank