The idiom Under false colours means if someone does something under false colours/colors, they try to show themselves something but they are not in order to betray people so that they can get success.
1. If a boat sails under false colours, it uses the flag of another country to cheat people.
2. Rajesh was under false colours, he never told me that he was a journalist.
3. Radha is sailing under false colors, she says that she is a republican, but endorses democratic legislation.