bolier pressure ?

I must admit I know nothing about boilers/central heating etc...but, we had a plumber out yesterday to fix a problem we had with our washing machine, and at the same time he adjusted the pressure on our boiler as he said it was too low (both the black and red hands on the pressure dial were pointing down at less than 1 bar. We also bled all of our radiators at the same time as they were all very cold. The radiators are now much hotter, but when we switch the bolier on the pressure goes right up, almost off the dial (over 4). What is the difference in the 2 hands on the dial? The red one is constant at about 1.5, is the black hand that goes up. Is this a problem?