I use dark blue Fortiflex "muck tubs" for paddock water, and have minimal slime problems despite being in a climate where *everything* grows. More than color, I think the smoothness of the plastic reduces algae growth. Where I used to board they had the black rubber tubs, which are rough-textured, and they were always slimy. I scrub all buckets once a week with dish detergent and a stiff kitchen brush (with a rounded head that gets into the corners nicely). That seems to really help also. The little (5 gal) buckets in the stalls almost never have slime, and the big ones only get a little bit.