In the stickies at the top of this section you'll see, fourth from top, a folder called 'consolidated helmet paint threads' (or something like that) and inside are numerous excellent paint up threads for helmets. They're all good. I'm a particular fan of Jayvee's and Terminal Fettler's but any of them will point you the right way.
As for paint, I'm currently using the Floquil acrylic paints for the back panels (Concrete, SP Lark Dark Gray, Silver, PRR Blue/Model Master Acryl Euro Dark Green), mandibles (Caboose, PRR Maroon, SP Lark Dark Gray, Silver, PRR Brunswick Green), ears (Silver, White, PRR Brunswick Green, Caboose, MM Acryl US Medium Green, Earth) and general damage (Silver and SP Lark Dark Gray). The base green for my current helmet builds will be Humbrol enamel #78. It'll be misted with varying shades of that color to create the aged and oxidized look of old paint.