On every major continent, there is now a Priest of Confession whom will assist players who have gone red from killing other players in redeeming their PK count. In order to summon an angel, you must provide 3 or more Sunstones and 3 or more Moonstones. Once you have summoned an angel, you must kill monsters and give 50% of the exp gained to the angel to prove your worth. Once you have provided enough exp to the angel (the amount needed varies on the type of angel summoned) it will be ready to be given back to the priest.
The White Angel requires you to give it 10% of 1 level's worth of exp.
The Green Angel requires you to give it 30% of 1 level's worth of exp.
The Blue Angel requires you to give it 60% of 1 level's worth of exp.
The Red Angel requires you to give it 1 level's worth of exp.