Download paint.net -
http://www.getpaint.net/and install.
Take the terrain.png out of the cartooncraft zipfile and open it with paint.net.
Zoom in, use the rectangle select to select the water on the bottom right...including the transparent space. Go up to 'Adjustments' and select 'Hue/Saturation'.
Adjust to likable color, click 'ok'. Save file, exit paint.net. Then put the terrrain.png back into the cartooncraft.zip, overwriting the old one.
You should be good to go if you do it right.