Well, I did it. I finally broke bad and ordered some artist quality casein paints but have not had much of a chance to work with them yet. I did paint this sunflower on a cradled hardboard but please don't judge too harshly as any painting in a new medium is not the best work that can be produced.
I really, really liked the craft casein I used on the cupcake in an earlier post. It was creamy and easy to work with. The Richardson/Shiva tube paints that I just got are much denser and very stiff and I have to learn how much water or emulsion should be added to get the consistency I need.
Also, I've found that different supports make a huge difference in how the paint behaves. The cradled hardboard used for the sunflower was gessoed and had some colors of the paint beading up on the surface. It evidently wasn't absorbent enough. I recently started another painting on 140# watercolor paper and the paint is sucked into the paper almost immediately - it's too absorbent! It's been hard to spread the paint much at all. Hopefully, there's a middle of the road support that's perfect.
Another issue that I need to work on is what type of brush to use with the casein paint. I was told that a decent synthetic was best but I'm finding the ones I have to be difficult to work with, especially on the paper support.
I'm hoping to find both the best brushes and best support for the way I work but it will take some further experimenting. I do like casein so far and think it may become a favorite medium once I learn to tame its quirks and exploit its good points!