I noticed something about colored triangulations of the topological sphere $S^2$ and have a question about this.

Observation.If you triangulate the sphere $S^2$ and color the vertices with three colors: then the number of 3-colored triangles is always even (or zero). In particular, there is no coloring with exactly one 3-colored triangle.

For a **proof**, view $S^2$ as two triangulated disks with matching coloring of the boundaries that are glued together. As their boundaries have the same number of color changes, we know from Sperner’s Lemma that their triangulations have the same number (mod 2) of 3-colored triangles. So the total number of 3-colored triangles is even or zero.

As an interesting corollary, we get the characterization: A triangulated sphere has zero 3-colored triangles **iff** all cycles of the triangulation have an even number of color changes.

I looked at the torus, the Klein bottle, and the projective plane, and I find that the observation is also true for them.

**Edit:** Just for contrast, adding an example below of a "soap bubble" surface, where the two soap bubbles share a common disk. This surface allows for triangulations with even *and odd* numbers of 3-colored triangles (but like the other surfaces I looked at, cannot have just one).

Question.I wonder whether this also follows from more general theorems about triangulations of surfaces, or about maximal planar graphs? I have consulted algebraic topology and graph theory texts, but could not find any results in that direction. Would you have a suggestion where else to look, or maybe a reference for that?

odd. $\endgroup$