Wet paint has a thin, smooth liquid film that reflects light more directly (like a gloss surface), which usually makes it look darker and more saturated than the same paint once it dries to its final, more diffuse-reflecting finish. Absorbent surfaces (like bare plaster) can also pull pigment and binder in unevenly compared with a sealed, non-absorbent surface.
Wet paint is covered by a smooth continuous liquid film, which reflects light directionally in the way a gloss surface does, and which also has a different refractive index from the dried binder. Both effects reduce the diffuse light returning to the eye, so the colour reads darker and more saturated. As the water or solvent leaves and the film develops its final microscopic texture, diffuse reflection increases and the colour lightens to its true value.
Matters most for emulsions and any matt finish, where the difference between the wet and dry appearance is largest. Matters most of all on absorbent surfaces, where drying is uneven and parts of a wall reach their final colour well before others.
Judging a sample — or worse, deciding to add another coat — while the paint is still wet. The colour at that moment is not the colour you have bought, and a coat added in response to it is a coat added for no reason.
Practical advice
Judge a paint colour only once it has fully dried and cured, not from the wet appearance — and expect a slightly different result on a very absorbent, unprimed surface than on a sealed one.
Always confirm with a physical sample in the actual finish before committing to a large surface — see methodology.