Preview images of the upcoming Spelljammer boxed set have been appearing on sites across the web, so I've gathered some of them together here. These also include the hobby store 'special edition' with Hydro74 art.
Think that started with 3E, could be wrong, but I think you are right about 5E. Thought they were few and far between, hard to find and rather small, not sure a Spelljammer could fit through them. But again I could be wrong, and Im sure if thats an option they'll be detailed in this set.
1e, actually. Color pools were in the 1e Manual of the Planes but IIRC they were first talked about in a Dragon magazine article about the Astral Plane. There was even a table in the MotP with all of the Outer Planes and what specific color of pool led there.
Typically that's how they were described, but they could be as big as the DM needed them to be. 4e explicitly brought the idea of "color curtains" into being - color pools large enough for ships and giant creatures to pass through.