I ran into this artist randomly, looking for ways on youtube to help me improve on my coloring skills. Her work is some of the best I've seen, and not to mention all of it is done with a MOUSE and usually all on ONE layer! (All my stuff is done with a mouse, but I'm not near as crazy talented as her) I hope you all have some time to check out some of her videos herehttp://www.youtube.com/profile?user=rtemilynguyen&view=videos OR you can visit her deviantART page here http://rinoatilmitt.deviantart.com/
I hope to one day get to the level as her, but until that day comes, all I can do is enjoy her wonderful work!!
This will help so much when it comes to shading and understanding photoshop, and I believe you can also use this with gimp. if you have problems using this tutorial with gimp just pm me! I'll be glad to help!
This tutorial helped me a lot when I started, although now I use my own method. Hope it helps you too!!
Here's a tutorial on coloring anime skin that I found to be very useful!
Layers are very very very helpful for those of you who want your colors to come out clean and in the lines. So, without futher ado, here is a tutorial I found that can show you how to use layers.
***You can also set the layer to multiply (you can find the scroll down menu at the top of the layer box) which will allow you to color over your line art and the color will go under the lines. So if you color outside the lines you can first of all see that you did, and second you can go through and erase it with the eraser tool or if you colored outside a large area you can use the lasso tool (I perfer the lasso tool on gimp over photoshop, so when using photoshop it's easier for me just to use the eraser tool), select the section by putting a dotted line around it with the tool and then hit delete.***