To evolve a card it has to first be level 50. You then need to sacrifice cards to evolve. 10 for a silver, 6 gold, 4 diamond, 2 legend and 1 epic.
Once a card is evolved it keeps its level 50 stats but is now evolved and level 41. Allowing you to level the card again back to 50. Making your level 50 card now have level 60 stats.
It cost the same # of rubies to go from 41 to 50 again that it cost to go from 41 to 50 when it wasn't evolved.
It also levels the same hp and atk per level. If you take a card and see the increase in stats for 10 levels you can multiply it out to project what it would be at higher or evolved levels.
Once a card is evolved you never get back the cards sacrificed or rubies invested to level the card from 40-50 arter evolved. If you reset an evolved card you get back rubies based on current level. Card is reset to level 1 but the card is still evolved. So essentially it is level 10 if you reset a card that was evolved once. It's level 50 if you reset a card that was evolved 5 times. You cannot unevolve a card. You can only reset its current level back to level 1.
Evolved colors go as follows.
1 = yellow, 2 = green, 3= blue, 4 = purple, 5 = red