Is swapping words within an idiom often done in chinese language? (example: "万绿丛中一点红" to "万红丛中一点绿")
Idioms are mostly elegant, memorable and meaningful phrases. Each character in an idiom has a part to play in depicting a vivid image or retold a historical story. 

It is difficult to swap any character in an idiom without making it sound forced. The phrase "萬綠叢中一點紅" depicts an image of a single eye- catching red flower among a field of green grass, that seem natural enough.  On the other hand, the image of "萬紅叢中一點綠" just does not seem natural. 


  Is swapping words within an idiom often done in chinese language?


The answer to your question is-- "rarely"

I can only come up with one decent example:

"鋤強扶弱" (beat down the powerful to support the helpless) is an idiom describes 'just and heroic deeds'. Quite a romantic notion. But this kind of hero in real life is very hard to find. In fact, you would see people bully the helpless to support the powerful more often. "鋤弱扶強" describe this behavior perfectly. 

Edit: Another one that would work

直言不諱- speak honestly, not avoiding taboo subjects

諱不直言- avoid taboo subjects, not speaking honestly