I had the good fortune to learn about color blending & matching from my Grandmother, an accomplished artist. She taught me how to work with oils & acrylic paints and that knowledge has served me well over the years.
First, I would suggest going to a craft store - Hobby Lobby or Michaels - and getting a variety of craft paints. Some are sold in packs with basic colors or you can buy individual bottles for < $1 typically.
Make sure to get at least one or two variations on brown, The ones I have are "Territorial Beige" and "Brown Oxide". ( If you can find it the Brown Oxide will already be close to the BNSF brown you need. )
Take a plastic bowl or plate that you don't care what happens to it - microwaveable meal bowls are good because they typically are divided and you can put water for clean-up in one of the sections. Put drops of the paint into the bowl, in this case I would start with Brown & Red, then dab a brush into each color and use a clear spot on the bowl to mix the colors.
Once you get a color that looks close apply some to a scrap piece of plastic and allow it to dry. Compare that to your car(s). If it appears close enough then use the mixed paint to make whatever fixes you need to make.
You may want to apply a coating of Testor's Dullcoat to the model after all the fixes are dry. That will even out any slight mismatches that may exist. ( Railroads themselves don't always get the paint perfect when making repairs. )
The key thing is to practice mixing your colors. One nice thing about craft paints is that they are inexpensive and clean up with water. So you can practice until you get comfortable with the process without a big mess.
Mark Gosdin