Cheshire Twins

Okay, I've never been more proud of my work before! I absolutely luv this and hope I did the concept justice. Can you tell which one is which?

They are the twins from Ouran High School Host Club, Kaoru and Hikaru. It's an amazing and hilarious manga so you should definitely check it out.

Even though Mori is my favorite, the twins are just pure awesome. I really liked their Cheshire cat outfits from the manga (not the anime) and wanted to draw them in the trees. I have spent a lot of time on this and am quite pleased with the result, which is very rare. The only thing getting me is the lines.

Can anyone tell me how to make the lines a different color, aka not black? I've looked at tutorials and they say to lock the lineart layer and then color it in, but I've tried to lock the layer with the two ways GIMP gives me, but it doesn't work. Am I looking in the wrong place or something? If someone could impart that knowledge to me it would be greatly appreciated, and then I could fix this piece and upload the better version!