Its an interesting question.
If an individual strand of hair does absorb water than this sounds plausable. But it partly depends on how fast hair absorbs water.
If the an individual hair is saturated on entry to the Pool then Chlorine would have to diffuse in which might be slower than theoriginal absorbtion.
This the nearest i can get to any real relevant science from the web
http://ppoa.org/?page_id=418
“Green hair in blondes is caused by too much chlorine.
Not at all. Green hair is caused by copper in the water – but not right away during a swim… Copper must precipitate as a green salt during a high-pH shampoo, usually long after the pool water has dried in the hair. Two errors, then: The operator had allowed copper (pipes, heater, impeller) to be dissolved by his excessively aggressive water. No problem yet – crystal clear water still. Error two is the swimmer’s fault; she or he didn’t shower (rinse) or even towel dry the pool-wet hair. It dried with that half cup of copper-bearing water leaving its contents behind. Then the high pH of a shampoo (pH 9 or higher), hours or days later, caused the precipitation of copper oxide (and maybe some sulfide). Everything from Aspirin to vinegar has been used to reduce the coloration after the fact, but prevention is the much better approach. The so-called swimmer’s shampoo is simply a lower pH product that doesn’t clean as well but keeps the hair below pH 8.3 where all the dissolved solids otherwise begin to fall out of solution. Everybody, by the way, gets green hair under this sequence of events; it just shows up better in bleached blondes.”
Hmm if this is a myth then its very widely repeated on the net and I’ve tried adding the word myth with nothing coming up. If its not true then its a harmless myth