Join the colours

Meaning : Enlist in the army

Orgin

Each regiment in the British Army had a flag, which called its colour. This dates back to at least the 16th century. In this citation from Certain discourses concerning the formes and effects of divers sorts of weapons, and other verie important matters militarie, 1590, Sir John Smythe appears to be disparaging about what must then have been new practise of called an ensign (flag) a colour:

"Colours is by them so fondlie & ignorantly given, as if they should (in stead of Ensignes) be asked how manie Colours of footmen there were in the Armie."

The word colour in this context is now best remembered via the annual ceremony of Trooping The Colour. In this, various regiments of the British Army, notably the Household Division, parade their regimental colour before the monarch.