I know that the Portuguese man o' war is a colonial organism composed of many "zooids", one of which is the pneumatophore, which essentially serves as a flotation device for the whole colony. This sac may be of varying shades of pink, violet, or blue.
I'm having trouble finding a reference on how a particular man o' war gets its sac coloration. The most obvious possibilities are trace chemicals in the air and/or water or heritable genetic factors, but I'm not finding any sources.
What determines the color of the pneumatophore sac of a Portuguese man o' war? Is it genetic, analogous to eye color or hair color in mammals? Is it essentially dyed by exposure to various chemicals in the environment? Is it gender-based? Is there some other explanation as to why one man o' war might be blue while another is pink?