The bucket is probably polypropylene or polyethylene. Paint doesn't like to stick to that. Rough up the surface with sandpaper, and leave it rough. The paint will stick, but won't be permanent. I did that to camo a bear bucket when canoeing in the BWCA. Worked well for a couple of trips, but flaked off eventually.
You could use hot glue to fasten black garbage bag or dark cloth around the bucket.