In a small sauce pan, add 1/2 the amount of water and stir in gelatin. Allow to sit for a few minutes.
In a separate bowl, mix together the other 1/2 of the water with cornstarch until dissolved.
Put the saucepan with the gelatin water over medium low heat until it begins to simmer.
Stir in cornstarch mixture and salt and bring back to simmer and allow mixture to thicken. Whisk mixture until smooth.
Turn off heat and pour liquid into separate containers.
Add a few drops of food coloring into each jar until you get desired color. Mix colors. Remember your elementary school days... red+blue = purple, yellow+red = orange, yellow+blue = green, and so on.
Notes
To use: Once the paints have cooled use them as you would finger paints. Place them in the fridge after use and you can use them again within 1-2 days.