Photoshop Night to Day

In today's lesson we covered day to night, we were given two images one of clouds and another of rocks. I started by changing the contrast and brightness, I wanted to darken the image and less the red colour of the rock, I chose a background layer of night sky with some clouds then went into the blend mod to blue and held down ALT, add a matte to the layer and painted black then I reduced the saturation, I still had a lot of red on the rock so I carried on reducing the saturation a problem with this was that the image didn't look as realistic, I had to go back a play around with the saturation for a bit I'm pretty happy with the results.