Grey primed, maroon from a car accessory shop can, roof painted grey and left to dry.
Cambridge Custom Transfers do a superb set of container decals and this is what was used. They come in one piece for the main details with separate British Railways branding and numbering for the ends.
Imagine lettering this lot from several pieces! A coat of acrylic varnish, which dries nice and quickly and there you have a completed container. Just have to decide wether to mount it on one of the mechanical Horse Trailers of make a new Conflat.