I’m happy to report back that the offload-reinstall-craft new easel method maybe sort-of worked.
It’s limited to using white or yellow as the third pigment though, and only works if I’m crafting in the original sp world the easel is crafted in.
I moved the new easel via portal chest to my other world where I was making an art exhibition, but it didn’t work there. It didn’t work in another world either, with white/yellow as third. But it works when it’s back into the original world.
As I was typing this reply it occurred that that world is actually the very first sp I created when first starting blockheads so I tested it out…. turns out even the other easels work there with white/yellow as third pigment. I haven’t played that world for awhile; choosing it after reinstall was a lucky fluke so I can’t confirm if it’s always semi-working in there or the offload-reinstall triggered it.
…this is simply bizarre. Still, I’m glad to have two more color options for the third pigment. Thanks once again to everyone, especially Milla, for bearing with me and for the all suggestions to try.