CS5 - How to replace the colour of the sky in one photo, with another?

Hi

I have two photos with different colour skies. What I'd like to do is replace the sky in one photo (which is greyish) to the same colour in another photo. I've tried masking the image so I can change the sky colour to the blue I want, but cannot find the correct hue.

I know it's tricky because the colour of the sky varies depending where the sun is, but is there a way to do this?

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Best Regards Steven Muster http://www.pbase.com/stevenmuster