It's pretty good, but it's not exactly what I mean by contrast.


As far as colors:
You can have colors fade into whatever you want. Blue can look better for shadows.
Different colors have a different feel.
red = passionate
yellow = energetic
blue = calm
Etc.

Saturation should lower at shades get darker or lighter. Your middle tone should have the most saturation.

It's always a good idea to change the hue as the brightness of a color changes.
When a object gets brighter, a certain color of light is shining on it. It could be red, blue, or some other color.

For example, if something is outside, you might want to make the lighter colors a little bit more blue because of the color of the sky.


Contrast:

When choosing colors you usually want shades from black to white for optimal contrast. Try to do it in 7 or so colors.

Use bright colors where the light is shining, and dark colors where it isn't. This is called shading, as I assume you know, but exaggerating this effect causes more (or less) contrast.


Highlights/Reflectivity:

When a material is really shiny, a lot of light reflects off of it. This is imitated in pixel art by highlights.

Metal, hair, beaks, plastic, water, (wet) skin, and other things of that nature are pretty shiny. So you add highlights.

Things like rubber, wood, or (dry) skin aren't very shiny.

Really, this is the holy grail of pixel art. Ever wanted to make something look metallic, but had no idea how? This is how.

You put highlights on the corners of boxes, or a point on a sphere that's pointing straight towards your light source.


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Ah, great, thanks. Gentlemen, listen to this gentleman. He knows what he's talking about.

That's a really nice tutorial, it inspired me a little. +rep