One vertex per element, one color per generator, with an edge from x to x·s.
(2,4)(9,10)(11,21)(12,26)(13,20)(14,17)(15,19)(16,23)(18,27)(22,36)(24,32)(25,30)(28,34)(29,33)(31,38)(35,37)(39,40) involution — undirected(5,6)(7,8)(9,11,13,23,14,24,15,25)(10,16,19,21,17,30,20,32)(12,22,27,37,28,38,29,39)(18,31,26,35,33,36,34,40)(1,2,3,4)(6,8)(10,17)(16,30)(18,33)(19,20)(21,32)(26,34)(31,36)(35,40)(9,12)(10,18)(11,22)(13,27)(14,28)(15,29)(16,31)(17,33)(19,26)(20,34)(21,35)(23,37)(24,38)(25,39)(30,36)(32,40) involution — undirectedA 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.