Let's for convenience N = Rm. Then, by the definition of Rm, the edges of KN-1 can be colored with m colors without generating a monochromatic triangle. Consider any such coloring. Pick any of the N-1 nodes of KN-1 and denote it P. Add an Nth point P' that will serve as P's clone in the following sense: P' is to be connected to all the points Q of KN-1, except P, and the new edges P'Q are to bear the color of PQ. Thus constructed graph has two properties:
This is practically KN with only one edge (PP') missing.
It does not have a monochromatic triangle.
The first property is obvious. The second is almost so. If there is a monochromatic triangle P'Q1Q2, triangle PQ1Q2 is also monochromatic which is impossible due to our selection of the coloring.