Some fans might have its own power cable and controler. Those i have used got dimmed when i turned down the fan speed. Getting a led cable/kit is probably a good idea.
yeah some fans have both power and led leads so that bypasses the dimming lights when turning down the fans (this happens because turning down fans decreases voltage thereby lowering power to leds as well) My cooler Master case fans (stock ones 230mm) have dual leads and I think Bitfenix is making some that way as well.