My gimp tutorial:
1. Make sure that the picture is in RGB mode. You can check this by going to 'Image' 'mode' and then 'RGB.'
2. Click on the little lasso tool. This will let you select the part of the picture you want to be in a certain colour. I tend to use 'feather edges' too but only at around 7.0.
3. Select the area you want to colour with the lasso. Remember to join the ends together at the end. Then go to 'colours' 'colourise' and it will turn blue with a little box at the top of your screen. Don't panic! You can adjust this. Drag the little slidey thing up and down depending on what colour you want the area to be.
4. Next adjust the saturation. For a hint, for skin tones I use 30 colour and 30 saturation. Don't adjust light and dark.
5. Have fun playing around. If you don't like how it turns out, there is always 'undo'.
6. Keep going until the picture has been completely coloured. I like to play around with levels, curves, contrasts, layers, textures and colour levels but that is a little complex. Message me if you would like to learn about those. Hope this helps and happy colouring!