There's more to it than that, folks. As originally delivered the CIE roundel wasn't only in white, it was slightly larger than the standard type - but most of all, the "tan" colour was exactly that - very significantly darker and browner than the standard "orange". This would be the most fundamental issue on a model based around the earliest days. Thus, one might call that the "as-delivered" livery, soon to be repainted into (standard) "supertrain" livery, and later (after addition of the "tippex white" stripes), IR livery.