So here's my pixel art from beginning to end. Obviously, I start off pretty rough and soften the image up as we go. The last image is my turtle so far. I'm hoping to make it more and more turtle-ee.

Good job so far. I like the cartoony appearance of the turtle.

The highlights being right next to the dark lines between each part of the shell looks jarring, and gives the shell a flat appearance. I would suggest lightening the outline where the highlights are.

Speaking of lighting, I am not sure where your light source is. The highlights on the shell make it seem like the light it coming from the top right, but the shadow on the back of the turtle's neck contradicts this.

Also, the green on the back of the neck and beneath the shell could be darkened a bit, maybe make it a midtone between the outline color and the body's color.

A mini update with some deets added

Actually, I think this is a little better: