I've found several tutorials here and other places that showed how to colorize a black and white picture with the GIMP. Most of them were convoluted and had too many steps so I decided to write one that should be easy for almost anyone to follow! Yes, it may seem that there are a lot of steps the first time through but once you've completed your first masterpiece you'll see it's really not that hard at all.
This is my first Instructable. I didn't want to submit one until I found an idea that would appeal to a lot of folks.
Step 1: Requirements
First off, you need The GIMP. http://www.gimp.org/
Secondly, you need your picture.
Thirdly, you need some patience (and a little good karma doesn't hurt!)
I was driving through Texas when I was this sign for Mean Gene's burgers. If you're a fan of old school professional wrestling you'll know Mean Gene. If you're not, then just know that Mean Gene was a legendary announcer in the professional wrestling world.
Step 2: Make Your Copies
Now that you've got the GIMP and your picture is open in it you need to make some duplicates. CTRL-D twice (or Image - Duplicate from the menu on the bar). You should have three pictures up now. You can minimize your original to avoid confusion if you want to, you won't be needing it.
I duplicate twice because I never work on my original picture, an old habit from the old days.
Step 3: Add a Transparent Layer
Ok, we're ready to start the process. On your FIRST duplicate, right click and go to Layer - Add Layer. In the box that pops up, rename the new layer as Middle (to avoid any confusion later on) and make sure that the Transparent radio button is clicked. Go ahead and click OK. You won't see any change to your picture but trust me, there's a new layer there.
Step 4: Time for Black and White
Now it's time to work on your second duplicate image. Click the second duplicate to make it active, then change the image to Black and White. Doesn't matter what method you use AFAIK, I happen to use the great script from FXFoundry. You can Desaturate, RGB/Greyscale, whatever turns your crank.
Once you're happy with your Black and White, go ahead and CTRL-C (Copy) it. Once you've got it copied you can close it out you won't be needing it anymore!
Step 5: Getting Down to Business
If you've made it this far you should have just one picture open (your first duplicate). Click on this picture and go to Edit - Paste - PASTE AS - NEW LAYER. Your duplicate should go to Black and White, with three layers showing in the Layers Dialog on the Toolbox (Clipboard, Middle, and Background). Make sure that the Clipboard layer is highlighted in the toolbox. If it's not, you''ll never see any results in the next step.
Step 6: Time for Some Color
Ok, it's time to find the color you've been looking for! Go ahead and zoom in on the area you want to color, click on the Eraser tool, pick your favorite brush, and go to town! This is where the patience and good karma come into play, to make your picture look good takes time.
Step 7: Finished!
Once you've colored to your hearts content, go ahead and save your new work of art! You've now got a fancy colored Black and White picture!