How I can change the colour shaded here to green, dark green, and light green in Gimp?
Here is another way to do it with GIMP to get dark green, green and light green instead of an undifferentiated green pattern like this.
Here are the settings I used to get an interesting effect with a variety of different shades of green.
Filters -> G'MIC -> Colors -> Channel processing
This exaggerates the contrast between the different shades of turquoise in the original picture.
The G'MIC GIMP Plugin does not come with the standard GIMP package, but it can be easily installed in Ubuntu from the Ubuntu Software Center where GMIC's package name is gimp-gmic. For other platforms G'MIC can be downloaded from the G'MIC website.
Colors -> Colorize
This changes the dominant color from turquoise to green.