Athena's Sprite Road.

[Gors]

Hoeloe nailed it.

Dithering is a technique to get midtones with two different colors, but it is hard to pull it off properly. In this case, the easiest is to simply add a third color.

Also recycling colors is important because this means you used the colors at its fullest. One of pixelart's characteristics is small color palette. This characteristic was inherited from old videogames which suffered from the same fact. Today you could make pixelart with thousands of colors, but it won't look good because:

1-it's hard to manage a lot of colors;
2-having a lot of colors means you'll have a lot of similar, pointless colors;
3-it will not look like pixelart.