Mmm, I think this is actually about the fresnel effect. And maybe even about absorbtion, but you can only manipulate transparency with that (in the shader tree, choose 'Transparancy' in the transparency channel, so you get the advanced options. And do set Refraction, to get the fresnel effect going.

But when you want this to make, for instance, 'flip flop' paint, that looks different in different directions, you want the 'fake fresnel' from Shader Ops by Digital Carvers Guild , which you can use as a blender in all kinds of shader constructions.

Hope this helps!