Well the black is the base coat I just used black spray paint for that but the yellow (I used citadel model paint but I'm not so sure that's the best one to use) needs a white layer of paint underneath first and I used masking tape that acted as a sort of stencil to get the hazard stripes.
You need to take the gun apart so you can spray the individual parts Id suggest looking up some guides on Google and YouTube before you attempt this so you know where to start and what should go where.
When I did the stripes I had to temporally stick the two halves of the housing together as the cylinder would have gotten in the way.
Its called dry brushing, you wipe the brush loaded with paint on a paper towel to get rid of some of the moister and lightly go round the model highlighting the places you would imaging it was get battered over time.
There is a few step by step guides on youtube and such (type "dry brushing paint metal") and like meany techniques it takes practice.