Look back through the story. What colours do you notice on most of the pages?
Can you find lots of yellow and orange? Are there any other colours you see lots of?
Draw an image from the story, use the colours you can see in the book. Focus on drawing sunflowers at first and you could try some others later.
You could use crayons - use the side of the crayon to draw in strokes. Use bright and vivid colours. You could do this with felt tip if you prefer.
If you have paint at home you could use it in a similar way to Van Gogh. Spread thick paint on the page and scratch into it with a knife to show texture and mix colours. This might be tricky at first so don’t worry if your first tries look a bit strange!