The first thing you want to do is add a multiresolution modifier to the cube and subdivide it 2 times. Then delete half of the square. Add a mirror modifier and check the box that says "Clipping."
Step 1: Shape the Head Part 1
Press "O" so that you can drag a point and the points around it. Press "g,z" and drag the point down. You also want to grab the two squares on the front and pull them in by pressing "g,y" and moving your mouse so that they move in. Press "O" again to turn off proportional editing. Then you want to select the four squares on the side (shown in the picture above,) of the head and pull them in by pressing "g,x" and moving your mouse so that the squares go slightly into the side of the head.
Step 2: Shaping the Head Part 2
Then you want to add another multiresolution and subdivide 2 times again and apply. Then go into sculpt mode and we will continue.
Step 3: Sculpting
For this part it is pretty much random. You will want to use the regular brush and raise the top and back of the head where hair is usually present. Change from regular brush to grab brush. Then turn off x symmetry by clicking on brush and clicking x symmetry. Now you want to grab the hair randomly and pull it out to give it the messed up effect. After that you want to add yet another multiresolution, switch to object mode, subdivide 2 times and apply.
Switch back to Regular brush and make your brush smaller. Then after turning X symmetry on again run your brush along the hair to raise parts of it. Add eyebrows and now is a good time to add facial hair if you want. Switch between brushes to also add facial features.
Step 4: Using Vertex Paint
Now you need to switch to vertex paint. Choose whatever color you wish for the hair. Now you want to run your mouse over the hair to color it. Continue doing this and switching between colors to add color to the face. If you can find a better way to add color be my guest, i feel that vertex paint isnt the way to go for small detail.
Step 5: Finishing Up
To finish up you need to switch to object mode and scroll down and check the Vertex paint box. Now when you render an image it will show the colors you painted on earlier. If you have any suggestions for a model in Blender or Lego please ask in the comments.