The word in question here is
ao. This word can refer to any colour ranging between blue and green. Both the sky and grass are
ao. In modern Japanese, however, it usually refers to blue; for green they have the word
midori. The kanji used to write the
ao in this case isn't the usual kanji, though. Here, the kanji 碧 is used, whereas
ao is normally written as 青. 碧 has a far stronger connection to green than 青 and can, in fact, even be pronounced as
midori.
Then there's the symbolism.
ao not only refers to colour, but also to anything young and youthful. In fact, not only is the sweat described as ao, Guy and Lee themselves are also called thus. They even use the same 碧 kanji. In English, the colour used for youthfulness is green. In the colour images we have of them, Kishimoto-sensei always colours Guy and Lee in pure, unambiguous green as well.
So although
ao can refer to both blue and green, and even refers to blue most of the time, it is quite clear that in this specific case, green was meant. —ShounenSuki