I’m not entirely sure what it is you want to do or expect, so:
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You can use the colour related tools in the Color tab to change the colours, but these changes will turn up as changed grey values. You can darken a blue sky by using the Colour Equalizer module’s Luminance equalizer for example.
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If you want to add actual colour into your black and white edit (make it sepia coloured for instance), you are limited to the B&W module.