One of my all time favourite watercolour projects! I teach this to Grade 7,8,9 and it gives them a good grasp of how colours work well together (analogous and warm vs. cool) and also brush control when using watercolours.
Students measure a grid onto heavy white paper. The smaller the grid, the longer and more challenging it is to paint essentially. So it's a good project for differentiation.
Then they draw an image onto the grid. I leave the subject matter up to them. They just need to make sure to fill the page effectively. Then they paint it using watercolours- I typically encourage warm versus cool colours but you could use any type of colour combinations really.
Some finished works: