A little discoloration and surface rust won’t hurt anything. WD40 or other oil would help protect and clean the discoloration. If your clamps are black and the black coating on the band gets scraped off, you could touch it up with Rustoleum paint. None of this should effect the strength or functioning of the clamps. 

Remember if you are paddling in salt water, rinse with fresh water and let dry when possible before packing the boat away.