Oil, acrylic, watercolour and gouache all suspend pigment in some kind of binder, but that binder, and specifically how it behaves as it dries, is where the four media genuinely diverge. Oil paint's binder dries extremely slowly, sometimes over days; acrylic's synthetic binder dries within minutes; watercolour's binder stays permanently re-wettable, letting dried paint be reactivated with water; and gouache sits between watercolour and denser paints, opaque but still water-reworkable. That single difference in drying time and reworkability shapes almost everything about how a painter working in each medium actually has to plan and execute a painting.
How long a paint stays workable determines how a painter can actually build a picture
Oil paint's slow drying time lets a painter blend colours directly on the canvas over an extended working session, softening edges and mixing tones wet-into-wet in a way that requires no rush, while acrylic's fast drying forces a painter to either work quickly to blend before it sets or deliberately build the painting up in separate, already-dried layers instead. Watercolour's translucency and permanent re-wettability push painting in yet another direction entirely, favouring a light-to-dark building process where white paper itself provides the painting's lightest values, since watercolour has no reliable way to paint light back over an already-dark passage the way an opaque medium can.
Choosing a medium is really choosing which working method a painting will actually require
Because these drying and reworkability differences are so fundamental, choosing a paint medium isn't primarily an aesthetic preference about final surface appearance; it's a genuinely practical choice about what working method a painting session will actually involve, whether mistakes can be easily painted over or corrected, and how much time a single sitting realistically allows for. A painter skilled in one medium often has to relearn real, substantial parts of their process when switching to another, precisely because the underlying binder behaviour, not just the visible colour, changes so much about how the painting actually gets built.
What we're still unsure about
That these four paint media differ fundamentally in binder drying behaviour, and that this difference drives real, distinct working methods for each, is well established, extensively documented studio practice going back centuries for the older media and decades for acrylic. What's more genuinely a matter of ongoing craft debate is exactly how directly techniques and working habits actually transfer between media for a working painter, since some skills, colour mixing judgement, compositional planning, transfer fairly directly, while others, timing and layering strategy specifically, don't transfer cleanly at all, and painters and teachers continue to hold differing views about how much cross-media technique genuinely carries over versus needing to be learned freshly for each medium.
This sits inside Paint Media (Oil, Acrylic, Watercolour, Gouache), one of seven topics in Painting, one of five domains in Art, one of seventeen subjects the app can quiz you on.