ventilation will dry it out.
Most likely the new plaster will be brownish when it first goes on and, as it dries, wil go light pink.
Any chocolate coloured patches will be where it has not dried out, usually on wet brickwork or where the plaster is particularly thich, as when it has filled holes in the wall. Paint is liable to peel off such patches.
When it is light pink all over you can rub it with a damp towel to to remove dust and give it a couple of mist coats. Mix them up in a bucket or spare paint tin, and stir that, and the tub you pour it from, very very very thoroughly.
10% water is about enough. On some very absorbent surfaces, such as concrete, I might use 20%.
The mist coat is supposed to soak in and practically disappear. The second will probably mostly lie on the surface. If not, mist it again so it seals the pores. You will know you have misted it enough when the wall no longer sucks the moisture off your brush. Mist coats should be dry in an hour. Open the windows, because ventilation is better for drying than heat is.
If you think the wall might still be damp (e.g. in a new house where the walls were exposed to rain before the roof went on) you can use a special non-vinyl porous paint such as Dulux Supermatt which allows the water vapour to escape through it. A couple of coats of this will make the wall look decent and you can overpaint it later. It is not very durable, but that is not its primary purpose. Other "new plaster" non-vinyl paints are available.
You can use matt white for your mist and base coats because it is cheaper than colours. It will also highlight to your eye any blemishes that you need to fill or smooth (and re-mist) before applying your expensive finish paint.