Its a tricky thing alan , as youve already noticed its quite prominent on a lot of jets so it always adds to the model if you can get it right. Just a couple of alternative methods you can try before splashing out £30 on umpteen different metal shades though. Coloured inks can be very effective , just a blue and brown , diluted and strategically washed over a silver base can give a nice effect ,particularly when talking about the cans on a flanker which have a distinctive blue stripy effect on the inboard sections. Another thing ive used quite a bit is eye shadow. Get a cheap palette containing as many different suitable colours ie. purples ,browns, blues ,coppers,bronze (poundland has some good ones and you can usually get all the colours you need in one palette) . I paint the exhausts first with a silver , then matt coat it to help the eye shadow stick. Then it a case of dabbing on the different colours with a cotton bud till you get the effect youre after ,if it goes wrong just use a baby wipe to clean it off and start again. Once youve got the effect seal it with a quick mist of hairspray or varnish but only sprayed, if you tried to brush it on it would smear off the eye shadow. Ive used this technique on a few models but particularly the f100 super sabre which had a notorious heat stained rear fuselage,heres a few pics cheers
tony