The evidence for an approximate "lepton as fourth colour" symmetry is so overhelming in the particle spectrum that Hanlon's razor does not seem to apply. Still, my own incompetence fails to recognise ...
What's the deepest reason why QCD bound states have integer electric charge, i.e. equal to an integer times the electron charge? Given that the quarks have the fractional electric charges they do, ...
In particle Physics it's usual to write the physical content of a Theory in adjoint representations of the Gauge group. For example: $24\rightarrow (8,1)_0\oplus (1,3)_0\oplus (1,1)_0\oplus ...