I do not know if anything has changed, because other pickup makers openly do it now, but I think you used to have to "special order" double cream, from Duncan. It was suppose to be due to Dimarzio trying corner the market (years ago) on the double creams, or something. Anyway, I know you can special order them, but Duncan used to want to put an unsoldered cover on it, prior to shipping. The new owner could simply slide the cover off, and have a double cream humbucker, and Duncan was merely selling a covered humbucker.....and was not responsible for whatever color the bobbins were underneathe.
Wish I had a more definite answer for ya..