It's an ill battle where the devil carries the colours.

The proverb condemns a fight or undertaking whose cause is represented by evil. The devil carrying the colours makes the banner a sign of what the battle serves.

It can also warn against joining a dispute on the strength of loyalty alone: before taking a side, consider the character of the cause that side is defending.

Use it to express moral disapproval of a dispute or campaign when its guiding cause appears corrupt. The image of a battle flag makes it especially apt when people are being asked to rally behind a side.