" ... to be painted no matter what they are made from ..."
Not always ...
USS HALSEY POWELL (DD-686) in February 1945, she has non-standard locations for her floater net baskets (relocated by the crew) and has a lot of stuff attached to the floats for survivors needs, like water canisters. The supply canisters are painted "blue", but I'm not sure that the "square-shape" floats are. They look pretty faded.
USS IOWA (BB-61) in about March 1943, doesn't exactly look like they match her 5-H Ms 22 paint. Maybe match her deck blue paint? I think they are black as they came. These are "square-shaped" floats as well.
USS NEW MEXICO (BB-40) in July 1944. Many battleships and cruisers laid out floater nets on the turret tops. The only case where I have seen "not dark" floats, but in this location should be "painted" deck blue, except that she is painted in dazzle. These floats are "cylinder-shaped".
USS NASHVILLE (CL-43) on 13 December 1944 after being ht by a Kamikaze. Not exactly matching her dazzle camo. Her floats are "cylinder-shaped" and they have attached survivor supply canisters as well.