There are several ways you can change the colour and contrast in an image, the first is using levels
Using the slider, you can change the amount of exposure, making the image either lighter or darker, by moving the left slider icon you can make the image darker, and by moving the far right slider you an make the image lighter.
the second way is by using the hue and saturation tab
Here, by moving the sliders you can make specific colours more prominent in the photograph, I have slid the hue slider to the blue section, this made the blue sky become more contrasted against the clouds
The third way you can change the colour and contrast is using the curves tab
Here you can pull the line around to change the contrast and brightness of the photograph. I found that creating an S shape with the line created a good amount of contrast in my image as you can see in the screenshot