Its the other way round Fraggle, the oldest ones started off brown, have a single slayer and have a name like "earth shatter" [earth elle slayer], then when you enhance them with gears they will get another slayer and turn black.
Then you have the next generation that where brown, had a single slayer but didnt have a name [they just say bow]. They will turn black and have two slayers when enhanced with gears.
Then you have the ones that still spawn now that just say bow and dont have a slayer on them but will when enhanced with gears, These can be fun when used with imbuing, ash enhance, then gear them

to make some nice 6 mod bows.
On the side of rarety i find the first generation bows that "havent" been enhanced yet the rarest as most out there have been enhanced [i pay 1mill for these as trying to get the set], all others i think are practually worthless.
Thunderz