Thanks @aghunter! I find colour balance a bit of challenge too, as even just setting a whitepoint on an image may not be good enough and you still end up with a cast. In that image I got a bit lazy and looked at couple of images of lightening on the web I liked and d tried to match the colour balance with the colour sliders. I think the clouds having a dark grey-blue to grey-purple seems to have the most effect. Also having the clouds a bit darker brings out the lightening a bit more.