This is an easy way to start thinking about color-mixing and is fun for all ages! Depending on their level, you can encourage counting and simple math.

Vocabulary: rows, columns, primary and secondary colors, parallel, perpendicular

Activity Time: 5 minutes

Ages: 3+

MATERIALS

Paper (preferably a heavier weight: watercolor paper, poster, cardstock or something similar)*, Watercolors, Wide brush**
*paper size does not matter, it can be a square or rectangle, big or small - creator's choice
**not essential but does help

INSTRUCTIONS

• Using a pencil, divide the paper into a 3x3 grid. 
• Break out the water colors! You will only need the PRIMARY colors: red, yellow, and blue.
• There should be 3 columns; paint them red, yellow, and blue. Rinse the brush out between columns/paints.
• Now paint the 3 rows red, yellow, and blue.

PROMPTS

• What happens when the red goes over the yellow? What color does it make? Is it different when the yellow goes over the red?
• You painted using only 3 colors. How many colors do you have now

NOTES
I tried this with both liquid watercolors and a cheap watercolor set pictured above. The watercolor set actually worked better in my opinion - the paper doesn't get as wet, which prevents it from excessive curling. I also recommend a wide paint brush - easier to cover more area and for littler hands to grasp.
The first time doing this, I thought I could save time by doing a column of red followed by a row of red instead of doing all the columns and then all the rows. You will get much better results (also easier to understand the exercise) by doing all the columns first, then the rows.