Often you can find a bit of the paint in an overspray area that was covered by some other piece that is well preserved, even if the original paint is totally lost. Start taking off bolts and look at the flecks of paint stuck to the underside of the bolt.

Also, we still have the techniques and materials used to make old paints. New batches can be whipped up (assuming they aren't too toxic) for comparison. Lead based paints obviously are more problematic, but modern technology is really good at replicating paint colors.