Cbreeze, I only have 4 tubes of oil colors: white, black, burnt sienna and yellow ochre. I know they are expensive and I'd advise you to buy the best you can afford, but they last forever (I bought those four tubes in the late '80s and they'll probably survive me!) Those four colours, mixed up or straight, are really all you need to weather most military models.
I also use artist's watercolors in tubes for washes though (black, sepia, burnt sienna, and white to make greys).