Finn isn't very hard to color.
Colors in this cartoon don't have much contrast.
So let's color his t-shirt first.
Make sure you use a lighter blue for the top than for the trousers.
For his backpack, use two shades of green.
Again, lighter on top and darker on the bottom.
Use light pink for the skin.
Then finish the outline.
You can use a white pencil to make the colors pop.
And now your drawing is done.