Whether paint gets lighter or darker when it dries depends on the type of paint that you’re using and the light that is in the room. This is because the paint finish often influences the perception of paint color, and the way the light hits the paint can alter the appearance of the color, also.
For instance, flat paint is a type of matte finish paint that has a chalky finish when it dries and is entirely non-reflective, so the paint will soak up any light directed at it. Generally speaking, matte paints remain very close to the swatch color for this reason and can make the color of the paint appear slightly lighter when it dries. However, it is important to note that all matte finishes are not the same.
On the other hand, semi-gloss and gloss finishes tend to typically make a color appear darker once it has dried because the sheen tends to reflect the light, although this is more true of gloss finishes in comparison to semi-gloss. Generally speaking, high gloss paint is far less forgiving and is nearly impossible to touch up without you having to repaint the entire surface.
It is also worth mentioning that the natural light in the room changes and can affect how the paint in your house looks at different times during the day as the seasons change. For instance, even though every wall in a room might be painted the same color, that color can appear differently on different surfaces, caused by the angle of the surface or where the light is shining on the surface.
If you are unsure of which finish of paint to use for a certain surface, I recommend that you do your research in accordance with the specific room that you intend to paint. For instance, a lot of people like flat finish paint on ceilings as it can hide imperfections with ease, as ceilings can be harder to reach in order to clean. On the other hand, a gloss finish can be an excellent choice for rooms such as the kitchen, as it is an easy finish to wipe clean when it comes to food spills or splatters, as this will just come off with a cloth.
That being said, you should take the type of paint and the light into consideration, especially when it comes to touching up a surface that you have already painted. The most important thing to remember is that if you are touching paint up, you need to make sure that you match the original color and type of paint you used on the surface with the paint that you’re using to touch up as I go into more depth below.