Hello,

I'm doing my first ever project in GIMP, and it's a map for an original setting I'm making. I've reached a barrier, however. My map is as follows:
Click image for larger version. 

Name:	YdwJeE4.png 
Views:	43 
Size:	4.08 MB 
ID:	112807

[For access to the multilayered GIMP file, see the following download link:
https://drive.google.com/open?id=1xt...aPvddYXy3IdkdM ]

I want to be able to paint portions of the map. So for example, I want to turn the top of the left continent white to reflect an arctic biome. However, I can't find a way to ONLY paint the green part. I could zoom in and carefully paint it white with a brush, but I feel like there's a tool to select portions of an image and recolour them. Do you have any guidance?