if you have Pro you can render the camrea view to texture, map it on a screen aligned quad, and use a simple pixel shader like:

float4 color=tex2d(entSkin1,tex.xy);
return 1-color;

this ought to do what you want.

or if you dont have Pro you can assign a material like this to every model and level texture.. and then have some variable that toggles between normal and inverted.