How to pronounce 绿林?
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.