How To: Use the Wireframe Node
The Wireframe node lets you colours the edges in a mesh.
1. Suppose we have a node tree like this:
Two Transparent BSDF nodes with different colours are mixed together with a Mix shader.
2. The Wireframe node drives the Fac setting of the Mix shader. For each point on the surface, if it is part of an edge then the Wireframe node will output 1.0; if there is no edge at that point it outputs 0.0. All edges therefore will be red (and partly transparent) while all other points are white, which in a Transparent node means completely transparent.
3. The result is this:
Note that you could easily use a Diffuse BSDF node instead of the second transparent node. If you do that, the edges would be brighter (because they would be opaque) but the shadows would be black instead of red: