Make sure there are no clumps in the cornstarch or arrowroot. You can either break them up with a fork in a bowl, or use a sieve.
Add the liquid soap and lemon juice and stir until smooth.
Separate the mixture into individual cups. A silicone tray works well too. You’ll want to have as many cups as you have colors.
Put a few drops of food coloring into each cup and stir well.
Use a paintbrush or fingers in the bath tub to paint the tub surround or tile with.
Wash off the wall with water to clean it off when done.
Notes
If you want to store these to use for later then opt for vinegar instead of fresh lemon juice. Once they've been in the bath tub and exposed to water they'll need used right away.