I decided to try this out with my children and see how it worked.
The children had a lot of fun making the paint, measuring the ingredients, stirring the pot, making the colours etc.
This is how the paint comes out when used on paper. Not very bright. I don't know if using different colour additives aside from food colouring makes a difference. The children still had fun using the paint which is the main thing.
Sadly though, within a week the paint was already going off inside the jars (stinky!) We would either need to use it in one hit, put much more salt in the recipe or keep the jars in the fridge (though it will still go off eventually).
* * *Since the children wanted to use some paint outside and the above jars of paint had gone off I decided to improvise. Using pieces of coloured chalk crushed in the mortar and pestle and adding a little bit of water to make a paste I made them some paint they could use outside. It was rustic yet they still had a lot of fun and it kept them occupied and playing together nicely for ages. Two of my favourite things!