Banff designs: difference methods for coloring incidence graphs

We present some results on the harmonious colorings of the Levi graph of a $2$-design, focusing on Steiner $2$-design. It is easily seen that the harmonious chromatic number of such a Levi graph is at least the number of points of the design: we study and construct Banff designs, that is designs such that this lower bound is attained.