The biggest problem with making custom (or reproduction) decals for use with trains of the miniature variety is getting the colors on the computer-generated decal to match the paint on the model. Eyeballing the color won’t work; you need a computer’s help to do it.
Enter a free Android app called Color Eye. Launch Color Eye, point your phone or tablet’s camera at the paint you need to match, and your phone will give you the closest color matches that it knows about, including a Pantone color and a CYMK value.
There are still dozens of things that can go wrong, of course. The match will be no better than your camera or your lighting. So for best results, zap your color sample under the same lighting conditions where you’ll use and/or display your model. If you have more than one phone or tablet, zap the sample with each (and borrow a friend’s, if you can) to get multiple opinions. And your printer may not print exactly the color combination that the computer sends it.
So even after you get a value, you’ll have to do some experimenting to get a perfect match. Using your software, create small swatches of any and all Android-generated color matches you can, then tweak the CMYK values a bit and create swatches of those as well, then print and compare. Once you find a match that works, use that as the background color for your decals.
When professionals do this, they use professionally calibrated monitors, cameras, and printers to get consistent results. With consumer-grade equipment, you’ll have to experiment to get good results, but at least with an Android device and this app, you can get in the ballpark.