Almost all OCs = Not matching the colour scheme of the show.
Usually: 1. The most common, black or dark colours to make thme look "cool"
2. Random colours that don't go well with eachother.
Their cutiemarks are either:
1. Something evil or dark to make them look "cool", most common
2. Bullets or a gun, same reason as 1 and also very common.
3. Their hobbies or intrests, sometimes common or uncommon it's 50/50
They usually have a stuffty backstory or some evil thing about them.
Then again not every OC is like this, I've seen some that actually fit in the show.