I found this easy and awesome tutorial on how to make bath paint over here at this blog. I will give you the instructions since they are easy.
– Take 1 cup baby wash (clear-ish in color – mine was yellow) and
– mix in 4Tbsp corn starch
– split that into 4 bottles (I used baby food jars)
– put in 2-3 drops of food coloring in each jar
– put on lid and SHAKE (it will separate between uses so just shake or mix before bath)