thieves raided supermarkets and electrical shops during 55-day spree .
four-man gang used pink umbrellas for disguise and fled in getaway cars .
they also threatened shop workers with hammers , knives and screwdrivers .
liam bell , 19 , marcus morgan , 21 , ashleigh evans , 26 and 22-year-old trea richardson , all from west midlands , pleaded guilty to conspiracy to rob .