I've used casein for about a year now. I still consider myself a beginner so I'm not sure how useful any painting tips I have with it might be for you. As for the paint itself I love it. I also paint with watercolor and acrylic but the first thing to note is that casein is not like either of these 2 mediums (it's not like oil either for that matter) in the way it handles, feel, etc. It really has its own learning curve.
One of the things I like about casein is that it is so flat matte and opaque. In fact, I don't think there's any other paint I've used that is as opaque or matte. It takes some getting used to for blending colors (like I'm used to with acrylic) but I'm getting better at it each time I use it. Right now I'm painting a picture of a clownfish with Shiva caseins.
I can't decide whether I like smell of the paint. I know I don't dislike it. It's just different. My boyfriend loves the smell of casein(which I thought rather funny). He always likes it when I start a new painting with them. He's walked in and picked up my palette,sniffed the paint and mumbled how much he likes the way they smell.
One thing I definitely recommend is buying both casein medium and casein varnish if they are available (they are in the US, don't know about for where you live though). I use the medium constantly - adding little bits at a time as needed along with small bits of water. Since Shiva caseins tend to be somewhat thick out of the tube they really improve the flow quite a bit.
I made the mistake one time of thinking that since a little bit of medium helped then maybe a lot would make them very fluid and transparent. The result was a disaster. It totally overwhelmed the paint and changed it into a kind of syrup. That was one of the few times I threw away the paint and had to start over.
One way caseins ARE like acrylics is that they dry very fast. Very very fast. I am buying one of those sponge lid-lock palettes just so I can keep them fresh. I've gotten tired of looking over to find they are starting to dry out on my plastic palette and adding water to keep them fresh (which sometimes has the unwanted side effect of diluting the paint too much for my liking).
One other thing to keep in mind about casein is that it is not nearly as forgiving about a support as acrylic is. It will crack if the support is not rigid once it has dried (or so I'm told). I've never painted casein on anything but Ampersand's Clayboard Textured but I'm sure any similar hardboard properly prepared with Gesso should do just as well. I've never had a problem with my casein paintings on the Clayboard.
It's hard for me not to be in favor of trying out casein. It is one of my favorite mediums. It is a shame they are not more popular here in the US. I only know of Shiva and one other small manufacturer (who's name I can't remember but who only supplies casein in gallon buckets) that sell it commercially.
I would suggest buying 3 primary colors and a white and maybe a black or brown to test it and see if you like it or not. It shouldn't cost very much. In the US at least, Shiva casein is quite a bit cheaper than equivalent watercolors, oils or acrylic paints.
If anyone else out there paints with casein and would like to correspond with me personally my email address is [email protected]
My AIM is mudmunk and ICQ is 24753847.
Best wishes on trying out casein!