It has been done, but I don't recommend it. There are many sources for natural pigments online, and in nature of course. "Box store" stuff doesn't always mean "cheaper" or if it does...it is just that...cheap! A little "searching" here at Permies.com will also provided a great many source conversations and discussions about methods and materials...
Commonly called 'fresco'. You are limited to certain colors, egg tempura provides the largest pallet (and EXTREMELY durable, 3000-4000 years) Certain colors, specifically in the blue spectrum, are not naturally alkaline enough to be coherent on cob.
Jay C. White Cloud
posted 4 years ago
Great points Chad, and I agree that most "pigmenting" get entirely too muted within the matrix of "cobb." In lime renders, plasters and mortars, as with paper plasters, you do get deeper color depth if you pigment the matrix of the plaster itself instead of painting.
I would point out also though, as Chad is alluding to, this does "fix" the color of the wall. With natural paints the colors can then change too over time with just the strokes of a brush, rag, or roller. I love milk, lime and tempera paints...they still haven't been beat no matter what "Benjamin Moore," "Sherman Williams," and other paint manufacturers would have us believe...Some natural paints may "chalk" or fad over the..."centuries"...but they sure as heck don't trap moisture in the material they are painted on and they don't cost a fortune when the "do start to bubble and peel off!"
Yes the iron and "natural" earth oxides in liquid form as in quickcrete off the shelf at the big box stores are the same as any powder form. Some have no or low VOC check the MSDS, just as the "low VOC paints" the natural earth and iron oxides cause the VOC or MVOC per some bad testing ASTM standard that misleads, even with earth oxides that can cause microbial and MVOC issues....try some test samples but yes the concrete oxides are perfectly fine best if no voc which most are. I dilute mine with water to get more out if it for the $$ they are highly concentrated. Local ceramic shops usually carry oxides that can be less expensive and just as effective, you may not need to go online or pay the shipping and pollute the atmosphere with shipping carbon monoxide. I think some need a better understanding of "natural". Ordering online causes more embodied energy than the production of OPC for example. The "internet" did good and bad to "nature" ...Local resources are best IF one knows how to utilize them.