In this video, in 3:00
The author suggests some scales in the circle of fifth are brighter than others. Despite having the same intervals. In the demonstration there, it does sounds brighter. So, In what sense is it true for scales? Why did the played chord progression become increasingly brighter?
This is a poor video, in my opinion. It seems to be oriented around some concept of 'brightness' that doesn't seem well-defined by the presenter, and doesn't quite seem to correspond to any other commonly agreed-on definition of 'brightness'.
He gets off to a bad start by claiming that a major chord sounds brighter than a minor one because it has a major third in it. Well, guess what - a minor chord also has an interval of a major third in it too!
He also attempts to claim that a consonant sound comes from it having nether too much nor too little 'brightness' - totally ignoring all the other more standard reasons that things are heard as consonant.
The idea that different modes of the major scale have different levels of brightness does perhaps have some coherence, but that doesn't support the idea that going around the major keys arranged in the circle of fifths makes things "brighter" or "darker".
There is also some logic to the idea that playing chord progressions that go round the circle of fifths can make a piece sound like it's going 'down' or 'up', but the way it works is not because you're genuinely going from a 'brighter' place to a 'darker' place (or vice-versa), but because the successive chord motions are interpreted by the ear as going downward or upward. It's rather like this famous illusion:
Over a small range of movement, going round the circle of fifths 'sounds like' you're going down (or up). But ultimately, you get back to where you came from (hence... 'circle').
There is a long history of the character of keys and scales. As ggcg says it comes before the time of equal temperature where the different keys had actually a specific sound (because of the pythagorean tuning.
Even the Greek had their theories about the character of the modes. This has been overtaken even in the medieval church music and later, when the names of the modes were the same but they weren't referring to the same hexachords anymore!
To me this question is really an opinion based thing. You'll find a lot of discussion also among composers in the last centuries.
In my opinion people who are swearing about this theory are influence by tradition (and associative learning) or it could be a certain function of perfect pitch.
Edit:
As phoog correctly says, the modes of the Greek were tetrachords, but the idea that certain keys had a special character has been derived and developed of those theories about modes. It is also correct that later a-minor and d-minor were built or C-Ionian and G-Ionian have been built on identical tetrachords. But the tuning of the Renaissance give them a special unique sound which was not an argument later when equal temperature came up.
Firstly, no two pianos are tuned exactly a like. Using one instrument to make a broad claim is not scientific. Even if a large sample of pianos were used, then the results would be valid for piano.
Now consider that different major keys are rooted at a different pitch. C# major is half a step higher than C major in the same octave range. Simply, higher is "brighter". Lots of pop songs shift key up a half or full step toward the end to create a bright mood shift, with all else being the same (chord progression, melody, rhythm).
Musicians often describe tones with less high frequency content (for instance treble knobs rolled off on amplifiers, or over-wound electric guitar pickups) as "dark" and their opposites are called "bright".
