I just wait till I get a model that requires some color or coating that I do not have in my collection, and buy it then. In some cases, I create the coating out of colors I already have. For instance, for bare steel, I prime part with dark grey (flat) primer, then drybrush with Testors silver enamel. This surface, and several other natural metal surfaces take on a patina that is mottled in appearance, so a single color does not give the right effect.