There's a very simple mixing device at http://painting.about.com/library/blpaint/blcolormixingpalette1.htm. I also have a Magic Palette Color Mixing guide from www.colorwheelco.com. Its very useful if you have the colors needed to mix: if not, you have to improvise. It's cheap and widely available on the web.
Putting google to work will help. Get specific: "how to mix Gray" or "monochromatic black" or "brown." There are a number of pages that work on that kind of question-answer format. I'm getting very interested in color mixing myself. And as noted above, there's a real learning curve here. But you might want to try the following - infallible no, but helpful I think. Go to a craft store and buy the prime colors of their cheapest acrylics - the kind of stuff designed for elementary schools. Pick up a 99cent plastic pallette while you're at it. This stuff is about the safest thing you'll find the world of crafts - you could eat it and it wouldn't do any harm. And dirt cheap. But color is color. Take a crack at a monochromatic black - once you've got a good one, you'll have a whole range of fantastic grays by adding a little white and or blue. Or mess around with red/white and a little green and you'll end up with a flesh or buff color: add in some more yellow and you're looking at something that would be sweet on a deck. If something doesn't work, wash it out. Clean-up is a few seconds and cost zilch. Kinda like finger painting so its fun. (BTW: this stuff will cover plastic is properly prepped but its cheap and the higher quality pigments of better paints of any type will show upon examination. Looks okay from 10 feet though.) BTW: I've got an old Compendium Guide to modeling armor done in Spain in 2000. Their modelers are very keen on oils for every phase of modeling. I'm going to give it a shot in the very near future: I've usually got more than one project going so drying time is no big deal.