This question comes up relatively often (and for good reason), and we thought it would be nice to have an explainer on the site to refer back to, so here it is:
The first and longest-used application of Roman Numerals in Centipede Press has been to denote Deluxe Editions, these being editions of generally un-fixed quantities in the 10-30 range (unlike a 26-letter scheme, for example) that are upgraded from the Standard Edition in any number of different ways. These can include combinations of better paper stock, larger size, and upgraded materials (usually leather in some part or the whole), and a traycase. They were much more common at the beginning of the Press and especially during the run as Millipede Press (where I think almost every book had a Deluxe counterpart).
"Standard vs. Deluxe Editions" is a post unto itself, for the future sometime. In terms of visibility across varieties of collectors, the most famous Deluxe Edition is probably the same as the most famous Standard Edition: Salem's Lot. Long hailed as one of the best treatments of any King novel,
and a grail to many King collectors, though at different orders of magnitude: a Standard Edition (left, not to be confused with the less-expensive and unsigned Gift Edition) is in the $3k+ range and a Deluxe Edition (right) is in the $15k+ range.
CBV on Salem's Lot Standard Edition:
CBV on Salem's Lot Deluxe Edition:
Most Roman Numerals are black. However, as we have grown to love about Centipede Press, there is no rule that isn't broken at some point in its multi-decade history, and this is no exception. In fact, the Roman Numeral Salem's Lot was the first with two different colors of Roman Numeral - 15 black (public sale) and 10 red (presumably all or most to King for distribution. This was back in the very first few books where the limitation pages were hand-numbered and not printed (that changed shortly thereafter). I'm certain there are other cases. I could see red Roman Numerals as the designation for contributor's copies (effectively PC copies) for a Deluxe Edition. But there are probably other reasons, too.