I made finger paint last week at Playcentre using those ingredients. Oh plus Lux flakes aswell. From memory the recipe was
3C cornflour
1/2C lux flakes
1 1/2C cold water
then add boiling water while rapidly mixing until it thickens into the consistency you are after.
Add a bit of food colouring
I made pink, orange and green but by the end of session the kids had mixed it all together and the lux flakes make it all chunky so it looked like a big mess of brown chunky

lol!