If you have worn a combat helmet for any length of time you will know that it gets taken off at every chance, dropped on the ground and generally treated rather badly. It is heavy, hot/cold and will constantly annoy you, but for all that, may save your life. A soldier generally has a love-hate relationship with it. If you lose it, then the enemy may be your second worst fear after the Quartermaster.
I am currently Painting a FJ force based on the Italian campaign in 1944, the miniatures are from BTD and really are full of character and easy to paint.
First step though for the helmets is choosing a base colour, for these guys it is Desert Yellow.
Second step is to wash the helmet and other metal parts with an Army Painter wash, this will dirty up the helmet and create some discoloration.
Third step is to cover up any of the washed areas you don't want too dark with the original colour being desert yellow, there is no hard and fast rule but generally leave it darker around the base. For this I have used the sponge technique, dip it in the paint and sponge most of it off, leaving a light coat in the sponge, then press the sponge on the helmet. This has an effect of 'randomness' that a brush can't really achieve.
Forth step is to use a lighter colour to create more variation, for this I used tan yellow and the sponge again but over a much smaller area. Think about where the sun may hit the helmet as a guide.
Last highlight for the helmet is with British uniform, just a few dabs here and there with the sponge and you should be getting a good gradual effect of dark to light colours making your helmet full of variation and interesting.
Lastly it is time for the chipping, helmets and other metal objects get knocked around so much that paint will eventually chip off. For this effect I use a panzer grey, it is a nice warm colour without being as strong as black. With your sponge dab as strongly or softly as you want to create the effect, keep it to the raised edges where you would expect the object to hit the ground, walls etc. Be aware that this colour will really stand out against your lighter helmet so do some test dabs first to make sure you are not going to really smother the helmet with your first touch.
There you have it, not only useful for helmets but also for objects such as fuel tanks for flamers or other metal type items. You can also do it with just about any colour but lighter colours will always stand out more, give it a try and let us know how you go! You can come to forum and chat about it here.