GIMP :: Colorizing Photos With Sheer Components

I've been playing around with colorizing black and white images, and it's mostly fine. Where I come in to problems, however, is when encountering a situation where something of one colour can be seen through something of another colour. For example:

This is very much the beginning of a rough draft, so I've not bothered being too precise with where I've coloured as yet, but I hope you can see the issue - the floor looks fine, the dress looks fine, but where you can see the floor through the dress, it doesn't look right at all.

At the moment, this is done by creating a separate layer for each colour, and setting that layer to "Overlay", but it doesn't matter what layer mode I use, I still encounter problems. Is there some technique I'm missing? I've tried playing around with the colours of the specific things - so, for example, changing the colour of the floor where it can be seen through the dress, but leaving it as it is elsewhere - but I've not found anything that produces satisfactory results, as yet.

Another related question is how is best to colorize photos like this. The method I usually use is to create a separate layer for each colour and then use a brush/eraser to paint in the part of the image I want coloured. I've seen tutorials, however, which say that you should fill a layer completely with colour and then use a mask to let only the bits you want to show through be visible. Is there any advantage to doing that? As far as I can tell, it achieves the same result, except that you can't then use Hue/Saturation to adjust the colour after the fact.