For me, finding suitable RR colors in rattle cans is always a challenge. I can get close on some colors by starting with rattle cans, then shading over it with thinned artists oils. It can be a slow process, due to the nature of oils. That got me wondering... would it be possible to start with clear lacquer, and add my own pigment? Has anyone else on here already gone down that road?
I have mixed pints together and them airbrushed them onto my models. Usually when mixing paints you start with a white base. For some colours they start with a yellow base. I haven’t seen it done with a clear base though.
But custom mixing, for me anyway, is kind of a hit and miss proposition.
No reason you cant mix your own, but as David says start with a white base for most colors and/or take an existing shade and add white, black or primary colors to it to tint. It’s a long trial and error process. My paint store guy does a drop or two at a time, then dries a drip on a white card with a hair dryer to see the dry color. When I try to mix my own I always end up with black (http://www.largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-surprised.gif)
Yup, I mixed a brown and yellow together once and got a pretty shade of gray. I was shooting for a darker yellow. So its best to go slow, a drop or two at a time, and sneak up on the colour you want.
Also, if its a colour you would want again, you should keep notes as to how much of this and how much of that to get where you want. Trying to match by eye, I usually end up a shade or two off. Of course, the real railroads ended up a shade or two off too.