One vertex per element, one color per generator, with an edge from x to x·s.
(1,2)(3,6)(4,5)(7,8)(9,10)(11,19)(12,15)(13,22)(14,18)(16,20)(17,23)(21,24)(25,26,31,37)(27,38,40,34)(28,32,39,33)(29,36,30,35)(2,5)(6,8)(9,11,13,12,14,20,23,21)(10,15,17,19,18,24,22,16)(25,27,30,39,31,40,29,28)(26,32,36,34,37,33,35,38)(11,20)(12,21)(15,24)(16,19)(25,27,29,28,31,40,30,39)(26,33,35,34,37,32,36,38)(1,3,4,7)(2,6,5,8)(9,12,13,20,14,21,23,11)(10,16,17,15,18,19,22,24)(25,28,29,40,31,39,30,27)(26,34,35,32,37,38,36,33)A Cayley diagram is a choice, not a fact about the group. Another drawing of this group — in a book, or on a different generating set — may look nothing like this one.