Water coloring, with either card stock or water color paper, is a favorite a lesson in our classroom. When this lesson is prepared in our classrooms, we have a basket (to hold the materials), a paint brush, paints (I usually remove black and brown), paper (pre-cut), and a water cup (There should be line drawn across the cup about 1/3 up. This line says, "stop don't go over me". It is a control for the students so they know how much water to get from the water source). Lastly, at school we use an "art mat" to protect the surface where the children are painting. These are vinyl rectangles; however as you can see, in the photo, use what is available in your home.
If you're introducing a new art lesson to your child, it's you're chance to establish where you would like art lessons to happen in your home. Do you want them at the kitchen counter or a specific table? Remember that toddler friends and preschool friends thrive on routine and understanding expectations. To work within this innate desire for organization, guide them to know where their art lessons can happen. In addition to knowing where to paint, it's great to have a place that you would would like your children to place their wet projects. At school, we have a drying rack; however, a piece of newspaper on the ground is fabulous. The idea is simply to have a predictable, consistent place to put the lesson when it's complete.
After each lesson is completed at school, the children return the lesson to its home. Every lesson has a specific place. If you want this painting lesson to be available to your child, prior to making it available, decide where the lesson will live in your home. If they know where the lesson lives, they can take the lesson from the shelf, complete the lesson and return it to the shelf.
Lastly, I found this idea in one of my favorite books called, "The Artful Year: Celebrating the Seasons and Holidays with Crafts and Recipes," by Jean Van't Hullabaloos.
https://www.amazon.com/Artful-Year-Celebrating-Recipes-Over-Activities/dp/1611801494