As you've noticed, there are H(ue) and S(aturation) controls on the palette.
The controls that are 'missing' are B(rightness) and opacity.
To select a darker tone (or lighter), I think you're supposed to use the 'Brightness effect' slider (in the same panel, once you click the button in the top right: 'show effect sliders'). The 'saturation effect' slider acts as a sort of opacity/mixing setting.
It is a bit clumsy if you are trying to choose, say, a very dark red directly from the palette, because you can't see the correct tone, but here's a tip to help:
Click anywhere on the palette and keep your finger down. When you move the dropper over the photo, it will select the appropriate colour in the palette (almost like a standard dropper; although it *still* doesn't select the tone for you). Anyway, if your target colour is in the photo already, this makes it easier to select.
Once you've chosen the colour in the photo, all you have to do then is experiment with the brightness and saturation sliders to get the right tone and mix. Again this seems a bit clumsy.
The only way I've found to do it is to paint on the photo and then move the sliders. Once I'm happy I'll undo the 'test' paint and start doing the adjustment properly. But remember, after painting you can always edit the 'adjustment layer', and so change the colour, brightness and saturation sliders.
