Anything goes in my world!
If mixing water colours, acrylics, enamel and oil paints gets the result I'm after then that's what I'll use.
The only important thing is to keep track on the previous layer because the next can dissolve it if you don't think about these things, like painting Tamiya paint on top of Vallejo paint with a brush will dissolve the Vallejo acrylics unless you've put some varnish on it.
I use what ever paint is the best match colour wise. As Jens wrote as long as you are carefull what you mix and match, and protect each type of paint from each other you'll be fine
I use what ever paint is the best match colour wise. As Jens wrote as long as you are carefull what you mix and match, and protect each type of paint from each other you'll be fine