As Cedi correctly noted, the shading of the leaves is based on the biome. There are two ways to change the local biome, but both involve additional programs:
Modding the game
A mod like WorldEdit can change biomes of the selected area. The relevant commands are:
/biome Get the biometype you are standing in.
/biomelist List the available biome types
//setbiome [-p] <biome type> Set the selected area to the specified biome type.
The -p flag changes the block (column) you are standing in.
Source: WorldEdit command reference
Video tutorial by "Flexo13"
Editing the world
An external world editor like MCEdit can set biomes on any part of the map using, for example SethBling's SetBiome filter. The forum post I linked also contains a video tutorial how to do it: