  We use Dirac quantization of flux to study fractional charges and axion
angles \theta in interacting topological insulators with gapless surface modes
protected by time-reversal symmetry. In interacting topological insulators,
there are two types of fractional axion angle due to conventional odd and
nontrivial even flux quantization at the boundary. On even flux quantization in
a gapped time reversal invariant system, we show that there is a halved quarter
fractional quantum Hall effect on the surface with Hall conductance of p/4q
e2/2h with p and q odd integers. The gapless surface modes can be characterized
by a nontrivial Z2 anomaly emerged from the even flux quantization. It is
suggested that the electron can be regarded as a bound state of fractionally
charged quarks confined by a nonabelian color gauge field on the Dirac
quantization of complex spinor fields.
