For understanding this image, you've to first learn what a Geoid is. The
geoid is the shape that the surface of the oceans would take under the influence of Earth's
gravitation and rotation alone, in the absence of other influences such as winds and tides. All points on that surface have the same
scalar potential—there is no difference in
potential energy between any two.(wiki)
Geoid - Wikipedia, the free encyclopedia
The colouring schematics have already been explained in the link accompanying that pic.
Besides, structurally also, our Earth isn't a perfectly spherical. It's swollen at the equator and slightly squashed at the poles, although it's not visually verifyable, owing to the relatively miniscule difference.
Strange but True: Earth Is Not Round - Scientific American