Not only skins are added, the new color schemes are used for creating new monsters which are a little better than ordinary one.
For example there is 10% chance that instead of regular Lava Troll the Magma Troll is spawned which has +30% better
stats (arm, str, hp and xp). There is also 1% chance of spawning Plasma Troll with +60% better stats.
Recolored monsters with weapons can be purchased. Other recolored monsters can be received in "other ways"

with game development.
If you want to create new skin check this thread:
viewtopic.php?f=2&p=72512&sid=866270bb6 ... 02f#p72512
Many thanks goes to kollex and DwarfTank who prepared many skins.