What stickers were included in early PM sets seems to vary quite a lot. Many sets were produced in their thousands, no doubt meaning multiple production runs. Geobra changed the packaging design quite often (leading to the most dedicated collectors seeking every package variant, even if the contents were unchanged; variants are referred to as
versions in
Collector) because that was relatively cheap to do, compared with creating new injection molds for plastic parts.
Stickers were a relatively cheap way of creating 'new' sets as well. Perhaps with a change of colour of some parts. I am thinking of the
early cars (several designs, relatively few molds, lots of sticker variation) and vans (I will post something on vans on PF in the next couple of days). For example, earlier (1977) versions of the ambulance in
set 3254x had 'AMBULANCE' and red cross stickers, whereas later versions (1982, at a guess) had the same stickers, but in addition 'MALTESER HILFSDIENST' and the logo (red Maltese cross) of Germany's largely voluntary medical aid service, giving owners of the later version the choice of which stickers to use.
In short, your memory of a set
without the coloured sticker may not be flawed, PlaymoGuardian!
Incidentally, my 3750 (bought in about 1998, though strictly speaking it is the equivalent set 3053) does have the coloured sticker. The set instructions (1997) on the PM website show both stickers and give the part number of the sticker sheet as 3088560, but earlier versions didn't necessarily have both.
I've just noticed that Corso's set box (second picture) shows a sticker sheet as part of the set contents, with a sticker of a framed picture of a sailing ship in colour, different from the later one, but no black and white picture sticker. So, as I said, stickers can vary widely!
Blue-sailed ship, now that I can't comment on!