Fukuoka may have used red powdered clay for an arbitrary reason- it was was the best available local clay. red powdered clay in his case was the local clay- he lived in the semitropical climate of shikoku, where red clay is abundant.
Ive made clay with local clay at 4 places between cali and washington, and suspect that color is not important, but powdering the clay IS important as it creates a more consistent ball that does not break apart in quickly rain, but melts over time. this would allow bals to roll a bit, distribuiting seeds, stop all the seed from being exposed at once to the hazards of being eaten, and allow some variability of germination across time that could mimic broadcast patterns of natural seeds- a several week spread is common in many plants, depending on where seeds land. Ill ask around on this, just to be sure im not making stuff up.
Not sure where you are, Eric - but here in Eugene I bought mine from Georgies. They cater to all the pottery folks around, but when I asked if they had red clay the guy said, "Like the kind to make seedballs with?"
I've used their clay in my kids classes and have been very happy with the results.
Just checked in with Larry Korn, who lived with Fukuoka for 2 years.
He writes "That's pretty much what I would have said. It helps if you add a little fine compost into the seedballs, maybe some lime if appropriate. Yes, it's best if you use clay from nearby, but not absolutely necessary. If you have trouble finding powdered clay you can always buy it at a potter's supply store.
There is another method, of course using wet clay. You mix it with the seeds then push it through chicken wire. Let it dry for a little while then roll it into little balls. Then let them dry out completely before sowing them."
Unless your flat without it, I'd use local clay from the ground. Perhaps getting in the car and buying mined clay is the only option you have, or the more practical one. With local clay, you can dry it and grind it through a scree to get it powdered.
Igoing to tru the wet clay method next time around. Fukuoka uses a cement mixer and carefully added the clay, compost and seeds, and the turned into little balls by being rolled in the machine. there are vids of this somewhere online, as well as plans for different seedball makers. I have always done it by hand, but will likely make a mixer this coming year...
If you use local clay, make certain that it isn't weedy. Clay is often from low lying or riparian areas, which also collect a significantly weedy seed bank from large areas of land. The latent upland weed seeds can then grow when you remove them from the wet conditions and seed bomb upland with them. So be careful if you are not using the clay right where you collected it! Don't seed bomb it far or you're sure to spread weeds.
Most elementary schools do not recycle their clay. You can plan ahead with the art teacher and take that clay off their hands. Donate some seed balls to the school in return! That's what I do with my seed balls business. The clay will have no seeds, and helps the environment by putting the clay to a green end, rather than landfilling it.