Isometric Pixel Art/Other Shapes
Now that you've shaded a cube, you can make other shapes (cones, rectangles, spheres) and shade them too.
Cones
Star the cone by drawing a flat-ish oval shape on the ground. Give the oval a center of one, and use the center to make the point of the cone. Connect the outermost sides of the oval to the point, and there's your cone. To shade it, use your four colors and the sun, add spots where the light is brightest.
Rectangles
Similar to cubes, just lengthen one side and shade it according to the sun's position. In the example, I added two colors on the same face of the rectangle that is facing the side, to give some depth as to where the sun is.
Spheres
For this, I drew a circle, then the sun, and shaded it by using lighter colors for the area closer to the sun. The example is shaded in a circular shape to give off that 3D look, and show it's a sphere.