Q:

How to pronounce 绿林?

The 绿林 were an agrarian rebellion movement during the Xin dynasty. How is 绿林 pronounced?

Wikipedia claimsclaimed that it's Lǜlín
But in dictionaries it's lùlín

Which one is right?

A:

It should be lùlín.
绿 is 多音字(polyphone) in Chinese, lǜ is usually used for color, lù is used for some 专有名词(proper noun).
绿林 is pronounced as lùlín, because it came from the name of the mountain 绿林山, which is pronounced as lùlínshān. 
And 绿林 should be pronounced as lùlín, it has become a proper noun, in general it won't be used for the meaning of "green woods".
There are also other examples of 绿 pronounced as lù. Such as 鸭绿江, 绿营.

A:

Like @songyuanyao said, lùlín is the one currently accepted in dictionaries. 
绿 and 录 used to share the same pronunciation, somewhat between lù and lǜ (let's for a moment forget about 入声). Then vowel shifts happen, in different ways and to different degrees in different times and places. Now 绿 and 录 are parted in mandarin, but not in some other dialects.
Historically, for whatever reason, mandarin tried to conform to local and historical pronunciations, that's why we see a lot of strange pronunciation for toponyms. 
But this trend is fading. Take 六安 for example, in newer dictionaries, it's read as liu'an instead of the traditional lu'an. The local people aren't content with this change, but there is nothing special with this 六; lu is just a local pronunciation, keeping it makes no sense. If we keep every local pronunciation, mandarin will explode with polyphones. 
Same case with 绿 in 绿林 etc. Whatever the context, 绿 is unmistakably color of the chlorophyll. Keeping two pronunciations will do no good but only confuse people and raise questions like this. If lù is one day erased from dictionaries, I wouldn't be surprised a little bit.

