'To troop the colours' definitions:

Definition of 'To troop the colours'

From: GCIDE

Troop \Troop\, v. t.
To troop the colors or To troop the colours (Mil.), in the British army, to perform a ceremony consisting essentially in carrying the colors, accompanied by the band and escort, slowly before the troops drawn up in single file and usually in a hollow square, as in London on the sovereign's birthday. [Webster 1913 Suppl.]