A few posts ago, I was bemoaning the lack of a consistent or correct Lehigh Valley Red. I then came across this model paint database on a forum. The on-line software allows you to upload a photograph, then match the closest commercially available model paint.
Of course you still have to account for the lighting at the time of the photograph, the age of the picture, the colour film used, the effect of the computer rendered colours and the effects of scale on your model.
I tried it for Lehigh Valley red, and of course, having uploaded a range of photographs, got a different colour each time – just like my models in the roundhouse a few posts ago. But it sure does give you a good start at identifying a colour.