Charter coaches are a mixed bag really, most now are formed of Mk2 coaches, but many still use Mk1 coaches and there are a few Mk3s kicking about too. Livery can also vary by rake, although there are few sets of coaches in BR liveries, any of the pre-grouping colours seem to be a good choice.
My advice is to look at pictures of charter trains in a couple of railway magazines to get an idea of what exactly is used. and go from there.