The most useful hour you can spend before repainting a room is spent doing nothing in it. Sit in the room in the morning, come back in the middle of the afternoon, come back again when the lamps are on, and each time look at the same wall. Almost everything that will later go wrong with a colour choice is visible in that hour.
Orientation is the first thing to establish, because it sets the character of the light for the life of the house. A window facing the sun receives direct light for part of the day, and direct light is warm, strong and mobile: it moves across the floor, throws hard shadows, and makes colours lighter and more saturated while it is on them. A window facing away from the sun receives only indirect skylight, which is cooler, weaker and almost completely even. It barely moves and it flattens everything slightly.
Those two conditions want different things. A room with direct light can carry deep and cool colours because the light supplies contrast and warmth of its own. A room with only indirect light generally cannot: cool colours there become flat and dim, while warmer and slightly deeper colours look considered rather than washed out. The instinct to paint a dark, indirectly lit room brilliant white to brighten it usually backfires, because white with no light on it simply reads as grey.
What is immediately outside the window matters nearly as much as which way it faces, and is much more often ignored. A tree close to the glass casts green light into the room for half the year and none for the other half. A red brick wall opposite bounces warm light in all year. A large paved area or a light-coloured building opposite throws a great deal of bright neutral light upward onto the ceiling. Stand at the window and look at what the light is arriving from, then hold a sheet of white paper up in the middle of the room and see what colour it is. That sheet is the truest reading of the room you will get.
Glass area and height change the distribution rather than the colour. One tall window throws light deep into a room and leaves the corners dark, producing strong contrast that suits a room you want to feel layered. Two smaller windows on the same wall produce flatter, more even light across the whole space. A window high in the wall lights the ceiling and the upper walls, which makes a room feel taller and leaves the floor level dim.
Once you have all of this written down, colour selection becomes a much shorter conversation. You are no longer asking what colour you like; you are asking what colour survives this particular light. That narrows the field quickly, and it is the reason two people can paint two rooms the same colour and only one of them is happy with it.
Finally, plan the artificial light in the same breath as the paint, not months later. Half the hours a room is used are lit by lamps, and a scheme that only works in daylight is a scheme that only works half the time. If the room is dim by nature, the answer is more sources of light at lower levels rather than one brighter fitting overhead: a lamp in the corner that was always dark, something bouncing off the ceiling, something at table height. Colour will do a great deal for a room, but it cannot supply light that is not there.