'To sail under false colours' means 'hypocrite'. This idiom has originated from the European navigational practices of the past centuries. Every ship had his own flags, while every country or group or even pirate gangs had their own flags and own colours too. This practice gave the ships a kind of identity. Clearly, sailing with a flag not of the sailor was bluffing identity.
1. We should not believe the politicians because they sail under false colours.
2. Sailing under false colours is not a good attitude for anybody.
3. His friends-circle is very less because he always sails under false colours.