There are several ways, starting with rattle cans. Pick a can that is is the region of the right colour, Army Painter or sum such. Halfords (just noticed your flag, so automotive) grey primer is also a good place to start as it gives a nice smooth surface to paint over. For the dark recesses a wash of the correct colour or just black will do the trick in most cases.
More involved is finding a friend who has an airbrush this will make the bulk of the colouring quick and easy, have a look in my gallery for my nids (they were an early foray into the world of airbrush) and Eldar.
Youtube is also your friend as there are many painting tutorials to give you ideas and inspiration as well as tips.