'Up to brighten, down to darken' is for sure partly true. Some exceptions: On most pianos I find F down to D makes it brighter, something about how D major sounds on the piano. And on stringed instruments or piano, the brightness partly depends on the chord voicings--a chord topped by the 3rd sounds brighter than the same chord topped by the higher 5th with the 3rd somewhere lower in the chord. Many variables affect sound and its perception.