This is a paid asset, but now you can download Paint Book Unity Complete Project Free.
Detail this asset from Unity Store: Original Link
Paint Book Unity Complete Project
“If possible, please buy the package to support the developer”
Choose the image that you like and color it the way that you want!
There are 3 images to choose from and after choosing one, you can start coloring it using a bucket! Just choose the color that you want and then select the part that you want to color. There are 20 colors to choose from, so be creative and color it the way that you want and like!