The days of British "Redcoat" infantry were very different to modern conflict- after all, they fired weapons that were highly innacurate, so massed volleys were the only way to ensure that you could hit the enemy. They also formed up in ranks and fired on other, similarly ranked troops.
Bright, distinctive uniforms in those days would help commanders to distinguish where their troops were. Higher quality uniforms would also send a message to the enemy that the force arrayed against them was probably well equipped (quality if not size or quantity of arms).