i've been catching em lately with red lips. they may be waitin for Jimmy Housten to catch em.maybe its the same reason flamingos are pink, cause they eat shrimp all day. Then you have other points of view. This one was based on a biologists study i found.
The colors you are witnessing are more to do with its state of activity and hormonal levels. If it is in prime breeding time - it will be good n ready to go blood pumping and full of excitement. It is likely that it's extra blood pumping you are witnessing making the mouth interior change color. The fish is over exerted. During breeding - all sorts of things happen to the fish extra strong colors develop - the tail becomes ragged and bloody, etc. Now; catch a bass in prime breeding form and in high stress from having just struggled with an angler. Note the color of the mouth
interior. Check again in half an hour and you will often see that it is paler.
Just as when we exercise, blood rushes throughout our bodies to supply increased levels of oxygen to the muscles and cells which require them -
so too does the same happen in fish. Where we get flushed looking cheeks from the workout - some fish can get the same effect in the mouth.
Who knows, could be a combination of the two theorys.