This is of course subjective, but over the years I've come up with my own ideas of those terms. Unfortunately, to describe it in words is a lot harder. I guess "dark" for me would be the kind of sound one gets when playing a note on a lower string at a higher position, "bright" would be the opposite. The E-string is the brightest string of all four. "Mellow" or "sweet" is sort of in between. "Warm" is sort of the opposite of metallic, it's a tone that sounds vibrant and alive. "Thin" means a sound lacks substance, has tone color close to like a squeal or a scratch. "Rich" or "Thick" is the opposite, it's a tone that's lush and full of substance. "Dry" often refers to off-string bowings and means the notes and short, crisp, and pointy. A note has "edge" when a certain part of the sound stands out. It can either be intrinsic to the tone, or forced by the player via, for example, accents. "Round" is the opposite, I guess, with no components of the sound standing out above the others. "Sharp" and "flat" are usually intonation terms and not referring to tone colors. One is too high and the other is too low. "A picture is worth a thousand words." In the case of tone, similarly it really defies verbal description; you just kind of have to be able to hear it.