I have run into the same problem. A couple of years ago I painted a CB&Q wood caboose a nice Chineese red, less than a year later it was pink.
With modern oil base paint, it seems that the factory made colors (ie Rustoleum and/or Valspar) resist fading much better than custom matched colors. With a little creativity, it is possible to get a nice boxcar red through mixing and matching the factory made red, brown and red metal primer colors.
Latex paints also seem to hold their color better, but just don't hold up very well on old wood unless extensive (and expensive) primer work is undertaken.
Jason Midyette