You can do whatever you want with any of the above kinds of paint, or else digitally. The most important distinctions between types of paint concern lasting qualities – the substrates can be tough once dried (like acrylic or oil-based paints) or can blur and wash away like watercolor and gouache when they’re re-wet. Also, cheaper paint will contain pigments that are called “fugitive”, meaning they won’t hold up under years of sunlight – you see houses sometimes that have been painted with cheap paint that’s faded over time.
I agree with @DigitalBlue – this was probably made with a digital watercolorish paint program.