I'd like to color the map using the "select by expression". I tried to use color_rgba(255, 0, 0, 255) for example. It only changes the color to be completely yellow which is the "selection color" in the general settings and project settings. So do I have to disable the selection color in the general settings somehow, or is there another way to use the color from the expression?
Edit: I basically want to color a map relative to the values of certain attributes. To give an example, I want to be able to color streets relative to their length with an expression which probably looks like the following: color_rgba(255, 0, 0, (length / max_length) * 255). Where would I specify such a coloring option?