Mixing Water Media

​Watercolor and gouache are so similar in formula, they intermix seamlessly, though doing so risks losing the full transparency of watercolor. High quality watercolors can be used as a pigment paste for distemper, casein and egg tempera, especially for students just wanting to explore the media without investing in a full palette of dry pigments. Small amounts of watercolor and gouache can be mixed into acrylics. but the higher the proportion of a persistent foreign binder like gum arabic, the greater the risk of compromising acrylic film strength. So, essentially, the main concern is avoiding interfering with the binder. Also, some pigments like genuine viridian can clot acrylics, so it’s a good idea to test before mixing a batch.

These are all intermixable with the caveats mentioned by Matthew. However, I find that mixtures like this often result in some compromise, at least in terms of traditional handling. It is certainly fine to use watercolors to experiment with other aqueous diluted mediums like distemper, egg tempera, etc. The making of aqueous pigment pastes can seem onerous and require a selection of dry pigments. This may seem daunting to someone who is not sure if they will be attracted to the new medium.
However, large amounts of gum Arabic bound (or dextrin bound like in the case of some gouaches) will compromise the water insolubility (or resistance) of a dried layer of egg tempera or even casein and glue bound distemper (the later tends to resist application of cold water at least if it is applied gingerly. Conversely, additions of acrylic dispersion media to paints that are normally reversible in water may provide some water hold out.
None of this is to say that the artist should avoid such mixtures, just that they should be aware of the affect such mixtures have on the solubility of media with a traditionally different solubility behavior.