A couple months ago I painted a series of quick studies from Howl's Moving Castle in Casein, Watercolor and Digital to compare the pluses and minuses of the different mediums.
Casein (first example) behaves like a slightly more forgiving gouache. It dries a lot darker so everything was a lot closer to the right value when they were wet. It has a nice smooth and chalky feel and probably (with a lot more practice) could get the closest to the poster paint of the original. Colors are totally off.
Watercolor (second example) has a lot more texture and I feel much more comfortable using it then casein. It is also harder to control. The clouds came out nice with fun edges but it was really hard getting it closer to the original. I had to settle with making the clouds look cool and not enough like the original. Colors are totally off.
Digital (third example) is the closest to the original and is the most forgiving. I could of gotten it even tighter but I wanted to keep it as a loose study. I did this one last and tried to get the cloud edges a little more painterly. Colors are the closest.
-Study-wise I need to slow down with traditional medium. I do most of my work digitally so I get a little to use to the control-z. My colors and values are off and I need to do swatches to be more accurate.
-What to bring to my professional work- I need to have some more brush strokes and texture in my digital work. I am sometimes too smooth and polished. I have already started working on that. I also need to be mindful of my edges. Sometimes I make everything too sharp.
With watercolor I just need to slow down and leave some more paper spots. I sometimes I try to be too close to my digital style and the painting gets overworked.
With Casein, I am not sure if I will do full scale illustrations with it. I like it and I will do more experiments with it but I need more practice...maybe grayscale next time. :)