which colours to use for liver coloured springer spaniel , I use faber castell pitt pastel pencils.?

I have done many dog potraits in the past using water colour but now find I get a better result using pastels, but I have just had 3 commissions come in for liver and white springer spaniels, and try as I might I cannot seem to replicated the 'liver ' colour, I have mixed reds and browns but still cannot get that lovely 'red brown'colour so can any one who uses these pastels help please, giving me the number on the pastels as they tend to go by numbers and not by the actual colour...Thank you