It needs to be understood in the context of the whole text. The word "positive" probably has two functions: evoking mood, and emphasising contrast with the surrounding winter landscape.
Explanation: Odd English, but I would understand it to mean that the leaves were not reddish or orange or any other color but yellow, so maybe pure yellow?
Just above a guess.
jccantrell United States Local time: 09:13 Native speaker of: English PRO pts in category: 36