I have been looking into colourisation (or colorization, as the USA-ians have it), and there seems to be two main approaches.
1) "Colorise and Mask"
Duplicate the photo to a layer, then create the colour by using "colourise" on the whole layer (i.e. the whole photo), then mask the layer so that only the bit you want shows; this is the Kizmiaz way, I think.
For example, for lips, one would duplicate the whole photo to a new layer, colour the layer red, then mask OUT everything but the lips (or mask IN the lips).
Having done this roughly, the colours can be adjusted by editing the colours on the layer (via HSB adjust) or altering the opacity of the layer, or both.
2) "Paint with transparent colour"
Create a new layer, and set the blend mode to "overlay" . The colour is applied "pure" on to the new layer, either by
a) use a paint tool (e.g. brush or pencil). This is quite intuitive, in that you're literally painting on the colour (but not the shades, this comes from the photo).
b) creating a selection (magic wand, lasso etc) and just flooding it with pure colour. This use of a selection is more similar to technique(1).
The tuning of the shades is then done using exactly the same approaches as in "colorise and mask"
This "paint on the colour" technique is well described here:
http://www.gimpinfo.org/tuts/colorizing-black-and-white-photos/I think this technique may be better than the (1) way, since it's easy to paint in TEXTURED or varied colour (such a tonal variation and highlights in the hair). It feels much more like using transparent water colour on a real photo.
However, I have only been googling colorizing techniques prior to doing it - I would welcome comment from people with experience.
BugBear