Masterpieces at Bath Time...a failure

Trying to find things for my 3 year old to do while I'm stuck on bed rest is always a challenge. He is one of those strange little boys that doesn't like to get his hands dirty. But today, Grandma offered to give him a bath, so I thought we'd try the homemade bath paints. That way he can get dirty, but instantly get clean again! Using ingredients I have in the house we made only two colors today, blue and yellow. Good thing we did, the original recipe called for what is listed below....

What you need:
1T cornstarch
1/8c. Baby shampoo
2-3 drops food color
1T water

Using this as my guideline, the paint was WAY too runny. It didn't even paint the wall! So we added 1T more of the cornstarch & it still wasn't thick enough. We ended up with 3T of cornstarch, which made it better, but what I am thinking for next time is omitting the water, and adding an equivalent amount of baby shampoo to the cornstarch. It was just too runny. The most fun of his bath was he loved making green and having his water turn green in the process. He had fun, even though he wasn't able to "paint" the tub.

I'll repost once we figure out a good balance.  I've seen the colored shaving cream paint too, but since shaving cream is rather expensive and we don't use it in our house, it's pointless for us to buy any.

You can see the yellow "paint" on the wall, but the blue was just too runny.