Big +1 on this from me.
Being in Scotland there are many, many varying colours of grey, white, black and blue in the sky at one time... being able to mask all these through scripting would be incredible.
Have also been looking for a way to only apply these masks in the top 50% of the image (along the horizon) so as not to mask all the important stuff with the same colour in the bottom half...
This could be achieved by either applying the masking to only the top 50%, or by doing the 'subtract selection' at the end to remove the masks that have been made in the bottom half.