Colloquial translation of "preaching to the choir"?
I would say the Chinese 4 characters idom "班门弄斧" could be best for "preaching to the choir".

“班门弄斧" basically means "display one's slight skill before an expert." "班" here represents the best carpenter in China ancient history whose name is '鲁班', '弄斧' means display somebody's axe skill.

Another one comes into my mind  is '关公面前耍大刀', '关公' was a big hero in China ancient history who can do '大刀broadsorwd' very well, and if you show your sword skill before him, that's really not a right thing.