This is going to sound very obvious but possibly because it’s so obvious, it’s something that’s often over looked. If you are choosing a paint colour for your walls remember to either pin the sample to your walls or hold it up vertically rather than look at the sample on a flat surface like a table. Why? Because walls are seen in slight shadow (as are ceilings) and the colour will therefore look darker on a wall than it will lying flat on a table where more light can bounce off its surface. This also applies to bathroom tiles. If for example you tile a bathroom wall and floor in the same tile, the walls will look darker than the floor.
It is particularly relevant when choosing curtain fabric. Fabric can appear much darker when hung especially when there are folds and ruffles casting shadows.
So remember if you are choosing finishes from a mood board don’t lay the board on a flat desk or a table.
The picture below shows the ceiling looking darker than the walls even although the same colour of paint was chosen for both.