Steve,
Is this the big four grouped companies liveries you are interested in? (GWR, LMS, LNER, SR) Or the pre-grouping companies which were their precursors which had an even wider range of livery colours? The simple fact is that these businesses had their paint mixed on a craft basis, and the know how was proprietary. One of the consequences of this has been a long-running and ongoing saga of arguments about exactly what colour certain items were painted!
The introduction of standardised colour scales with samples (such as the well known Pantone references) long post dates the development and routine use of these railway livery colours, and it would be interesting to know if any credible matching has been undertaken; but I don't presently know of a site with such references.