One thing I have done to try to match paint colors to decals is a bit involved, but I find it a help. I scan the decal, and paint a piece of plastic with the paint I have mixed. I put both samples into the scanner to get them both in the same scan.
I then scan the samples, and look at the scan file with my photo editing software. Most of the software (Photoshop, Elements, Paint Shop Pro, etc) will allow you to sample a color (eye dropper function) and will give you a measurement of the color in terms of red, green, and blue, or in terms of saturation and hue. You can then see how close you are and which direction to go.
To use this info, you need a bit of knowledge of color theory, but if you are mixing paint you should brush up on that anyway. Check out a library book on painting- most of those books go into the theory of color and paint mixing. And, stop at an art store and pick up a color wheel or color chart- they are not that expensive.