- RYB mixing (e.g. yellow painting plus blue paint = green). I tested it on a mix of alizarin crimson plus hansa yellow to produce a cold orange (e.g. blood orange).
- Relative behavior of specific pigments. For example, a Prussian blue is almost black when applied thickly, but very chromatic when applied as thinly over white. A cerulean blue is pretty much the same in those two states. I tested this on a Prussian and the app performed well.
The app seems to make the assumption that the artist will always want the paint to mix. In a glaze, a thin transparent layer of paint is applied over dry paint. Effectively the result is a filter (the result will always be darker). To a degree, this can be faked using paint applied in a layer the blend mode of which has been set to multiply. It might be nice to add a drying brush to the tool set which selectively dries the paint to prevent mixing. Maybe also a mode switch to selectively different modes of physical application such as glazing, scumbling and dry brushing. While we are at it, why not add a Tonk mode using which thick paint can be lifted off the surface without smudging.
When I paint digitally, I mix the incredible nuance of real paint... even its smell. When I paint with oils I miss the incredible flexibility of the digital process (blend modes, compositing, even the humble undo). Never the twain...