(shading, flat-shading, gouraud-shading, interpolative-shading) Flat Shading.

The same color to an object in order to represent the effects of light. All pixels inside a polygon are given the same shade. Interpolative Shading.

Each pixel inside a polygon is given a particular shade, which is determined by interpolating between the polygon's vertices, or between the polygon's edges. Gouraud Shading.

Gouraud shading is a method in which the triangle color is obtained by interpolating the vertex colors that are located at each corner of the triangle. By utilizing this technique, 3D objects appear increasingly realistic due to the smooth, curved appearance of the surfaces, even though they consist of many separate polygons.

There are some better algorithms: Phong and Metal Shading but both require, at this time, too much calculation power to be used in real time.