You mention using stain on the earth walls. Is that similar to the stain that is used for concrete
There are many types of concrete and earth staining products on the market today all with different chemical blends. We use a variety of high-end acid stains that are very effective and not something you want to touch directly, but we wear gloves and masks and are very careful with the acid. One interesting thing to note about the toxicity of the acid stain is that earth wall itself is stabilized with cement and this creates a slight alkalinity which the acid stains can neutralize.