Most songs are written by songwriters in a key that matches the color of that song and then they go search for a singer with the vocal range of that song.
If you play all the chords there are ( all 12 ) then you'll notice that some of them sound bright and some darl so actually you could think of it as a color chart for choosing paint, for instance the keys of E and A sound bright as in a bright yellow or green color whereas the key of Bb for instance sounds dark as in a black color.
So if you transpose a song into an other key you change the color and also the mood of that song and especially for good written songs like evergreens can destroy song entirely.
Herman