I'll try not make any blanket statements about them.
There are four species of blanket octopuses in total. What sets them apart (aside from the colorful webbing) is that the females are gigantic when compared to the males, with the former reaching lengths of up to 2 meters (6 feet) and the latter only 2,4 cm (less than an inch).
So yeah, only the girls sport the rainbow membrane. And it's actually used as defense mechanism: when threatened, they will unfurl it to make themselves look bigger.
There are four species of blanket octopuses in total. What sets them apart (aside from the colorful webbing) is that the females are gigantic when compared to the males, with the former reaching lengths of up to 2 meters (6 feet) and the latter only 2,4 cm (less than an inch).
So yeah, only the girls sport the rainbow membrane. And it's actually used as defense mechanism: when threatened, they will unfurl it to make themselves look bigger.