Well, If you have successfully opened the image in GIMP:
open logo.png and..
1 In the window of image, from the menu (file..Edit..Image..Color..etc)
select:>color> Hue-saturation.
2 three sliding controls will show up.
Hue: it will set what color the image has. slide it and you will see the color change.
so, simply get right color you want.
saturation: how much color, black n white to full color.
lightness: its the brightness control.
3. Now when you're satisfied with colors, click ok.
4. click>File>save or save as.. Done 