B4, really. It just has that hint of more contrast. For a flat shaded version this is fine, for a more realistic shaded version, you shouldn't go as you have done so far, but just add surface and texture information in the shading transitions and add a slight bit of contrast in the darkest and deepest part. It shouldn't be all smothed out with 4 more colours, actually 1 or 2 would be enough if you apply it to define more information, rather than more smoothness.