Lighting shader usually use normals. As you may remember from the last tutorial, normals are the vectors which are perpendicular to each triangle. I will explain them in better detail.
gl_FragColor = vec4(v_vNormal*0.5+0.5,1.0);
Because normals are in the range of -1 and 1 we'll multiply it be 0.5 and add 0.5 to put it in the range of 0 to 1 (since we can't see negative colors).
Here is what it looks like (downloads at the bottom):