Hmmm.....well, my belief is that original coloured vinyl in America, during the 50s and 60s was made for radio DJs, rather than for public domain. The reason being is that DJs would have to rake through pile upon pile of new releases every week for their radio show, and the coloured vinyl promos were a way of making the records stand out amongst the mundane black vinyl promos. The hope was that, because they looked attractive, they would encourage DJs to pay more attention to them, and give them airplay simply because they looked cute.
There is another possible side to it. Remember that demos and promos were almost, always pressed on virgin vinyl, rather than recycled vinyl, which was used for a lot of stock copies, since the sound quality would have been ten times better, particularly for broadcast purposes. The same can also be said for coloured vinyl; sound quality appears to be a lot better.
I've come across boxes of doo-wop boots & reissues in rock & roll/swing shops, and quite a sizeable amount of these had been pressed on coloured vinyl. However, it's easy to tell that they're boots, mainly due to the way the grooves look, and also checking the slapdash quality of any run-out markings.
Other than that, I wonder if the odd coloured vinyl stock copy was the work of a pressing plant worker, who either pressed a few up for his own collection, or wanted to brighten up a boring day at the plant by pouring coloured crystals in the vat just to see what came out? Perhaps it was one or two of these that just happened to leak out from his hands. Certainly looking at Tefteller's website (whcih Miss Popcorn has kindly provided a link for on here), it's amazing to see how many of the coloured vinyl pressings were "found at the home of a pressing plant worker in 1999"!
Hope this helps you Ged, and hope I haven't gone off into a tangent!
Gene