I have a mesh that should be yellow on both sides, front and black, yet only the front side is yellow, my back side is fully black. How can I tell it that both faces should have the same colour? I tried to use the property back face culling but it doesn't do anything.
Well by default all faces have only one normal. A normal indicates the direction that a face is pointing in. In general 3D it is usually not possible to have the same face show different colors. For more on normal's read https://www.blender.org/manual/modeling/meshes/editing/normals.html
Even if you have cloth you have a back face and a front face of cloth it may be paper thin but it still possesses thickness. A butterfly wing has some thickness too and the front and back should usually be modeled as separate faces this is important if you are going to use the model in another engine or you want it to work anywhere.
But certain render engines provide that feature where the reverse side to the faces normal can be a different color.
Blender cycles provides that feature but it is part of the materials.Blender internal also provides that feature but its implementation is different.
I guess you can use this feature if your model is limited to cycles otherwise create another face for the butterfly wing.
You can test the above node setup by Just adding a plane and adding a cycles material to it and using the above node setup (rotate the plane and and you'll get the idea).