I am afraid not. :c)

My feedback is just based upon thinking things through.

that being said - before filling something in that has 3 dimensionality - consider where shadows (or semi-lighting) could be placed to enhance depth of field. In this case the inside of the skull could have "cheated" with a dim shading of the inner skill (perhaps light bounces upon the bone-white/yellow surface (scattering effect)).

As with all things - practice makes perfect - and it doesn't hurt to look at art shading to see how other artists do it. :c)