The visual impact of large groups in bright uniforms messes with the heads of witnesses. Seeing all the other people wearing the same shirt as you are makes you feel solidarity with the group. You are not alone. We surround them.
And it scares the bejesus out of the Other Side.
Giuseppe Garibaldi, the Italian revolutionary, made the Camicie rosse famous
The Mississippi Rifles, Buena Vista, Mexico -- February 23, 1847
Firefighters attack a fire at Murray and Church streets, Manhattan, on Sept. 9, 1861. Firemen made the red bib front shirt popular
National United Front of Democracy Against Dictatorship (UDD) (แนวร่วมประชาธิปไตยต่อต้านเผด็จการแห่งชาติ)
UPDATE 20090428: Red shirt Fridays