On the chromatic number of Steiner triple systems of order 25

It is well known that a Steiner triple system (STS) on v points, an STS(v), can be seen as a collection of 3-subsets of its point set, called triples, such that each pair of points occurs in exactly one triple as well as a decomposition of a complete graph on v vertices into cycles of length 3. By combining those two views on STSs we develop a method for its weak coloring. This method enables us to show that each STS(25) is 4-colorable, which in turn implies that the chromatic spectrum of STS(25) is {3,4}˙.