Viologens, in their dicationic form, typically undergo two one-electron reductions.  The first reduction affords the deeply colored radical cation:
: [V]  +  e     [V]
The radical cations are blue for 4,4-viologens and green for 2,2-derivatives. The second reduction yields a yellow quinoid compounds:
: [V]  +  e     [V]
The electron transfer is fast because the redox process induces little structural change. The redox is highly reversible.  These reagents are relatively inexpensive among redox-active organic compounds.  They are convenient colorimetric reagents for biochemical redox reactions.