Ah, now to add the detail to your outline.
First make a palette of the colors you are going to use, and for each color, make a darker version of it, and a lighter version.

Now you need to start coloring your outline. avoid using the darker and lighter version of each color.

Once your sprite is completely colored, now is the time to use the dark and light colors.
It is often difficult to utilize these effectivly.
You can do 1 of 2 things.

The first, is color in the entire left side of each section with the darker color, and the entire right side with the lighter color.

The second is to color the bottom left portion of each section with the darker color and the top right portion with the lighter color.

You should experiment between the two to see which you think looks better. And when your done, your sprite will look great and with the darker and lighter colors, it will look shaded and will enhance the viewability of it.

Good Luck with all of your future sprites,
and I hope this helped all of you.