Great project, Ben. I am planning on working with IBCs soon to expand my aquaponics setup from 50 gallons to 250.
If you plan on using the water only on your garden, you may want to
encourage algae growth. As it breaks down, it will provide a low level of nutrition to your plants. The USDA is studying using algae to
treat waste water from cattle operations and using the resulting dried algae as an organic fertilizer. The amount of algae that your system adds will be low (unless you start to add nutrients to the water to encourage algae growth), but over a growing season it will help your plants.
I was reading up on IBCs and found that the plastic is highly UV resistant, lasting 20 years or so in direct sunlight. If you are not worried about algae there is no reason to paint the container. The only issue you may encounter is trouble with clogging, but this would require really high algae concentrations. However, a toilet brush on the end of a broom should be enough to clean out a clogged drain.
Now you've got me excited about collecting rain water and growing algae
.