I have a 500 gal. converted rain barrel that I plan to use for collecting rain. It is made out of thick walled translucent plastic. My question is should I paint the barrel a dark color to prevent algae build-up inside the tank? Thank you in advance.
There's two separate things - translucent, and light/dark. If it were me, I'd go with paint, yes, to block light from algae. But I'd make it white rather than black so as not to heat the water so much in the summer.
I agree with realbirdlady; keep out the light but use lighter paint that won't raise the temperature of the water inside and encourage algae growth. My plastic rain barrels came in a light green color.
I really doesn't matter if you paint your barrel. If you paint the outside with an acrylic enamel, (Test a little area around the top rim) make it a light color that blends with the landscaping. This will help prevent UV deterioration of that fine and EXPENSIVE chemical barrel and stand.
If the top is open, algae will find their way into the water and thrive, because rainwater contains dissolved atmospheric nitrogen. If you are worried about algae buildup try any of the algaecides available from pond supply companies that is safe for fish. There are products that are nothing more than blue dye which are very effective in controlling algae growth in wastewater treatment lagoons. They stop ultraviolet transmission and thus photosynthesis killing algae. Yet they have no apparent harmful effects on fish & bacteria. The dye products are EPA approved, but check for warnings about use around your garden.
Don't use Copper Sulfate or Bleach to kill algae as it is overkill.