From my experience, I would suggest placing the water tanks underneath the shed roof. When placed in the sun, the water stored will very rapidly grow algie which will turn it green and cause an odor. I paint the outside of mine to block sunlight, however it still happens. I am in the southern USA, so we do have summer temperatures in the upper 90's regularly.
I have two tanks that are under a roof cover, plus are painted, and I still have to treat them with a small amount of bleach from time to time to kill the algie under control.
