Earthbag projects that take more than a few weeks to build will need to be protected from the sun. You could cover walls with tarps or coat the bags or tubes with latex paint. Or you could order UV-rated bags or tubes that are specially coated to protect against sunlight. The higher the UV rating, the longer these products will last in direct sunlight.
Here’s one company offering bags with a 1,600 UV (ultra violet) rating — Commercial Bag and Supply Company. I found one company with a 2,000 UV rating, although they don’t carry standard 18”x30” bags. Pranavi Polymers Pvt. Ltd. makes UV-rated bags to order. Use Google to find companies who sell standard sized UV-rated bags near you. Bags like this can save a lot of work and protect your structure from UV degradation.