Gradually, I started adding more detail to the figure and the background by using pixel dithering to achieve definition and dimension. Dithering is when you "checker" the pixels in order to smooth one color into another. This is a useful technique that allows you to use a limited color palette and still achieve dimension.
Limiting your color palette is important in pixel art. Old video games weren't very powerful and could only use a limited amount of colors. While that was out of necessity back then because of technological limitations, contemporary pixel art tends to adhere to these traditions. If you have too many colors or "steps" between one shape and another, it ends up just looking like a blurry photo instead of sharp, clean pixel art.