When it's still dark and the sun is starting to rise if you expose for the sky and try to get the colourful light as it appears you will find the darker parts of the frame might get even darker as they turn into silhouettes. The difference between the dark foreground and bright skies is hard for a camera to capture, your light meter that helps the camera gauge light will adjust the exposure based on where you point the camera. In the case of manual settings you will get a reading at the bottom of the viewfinder that tells you whether you will be over exposed or under exposed. You can use this to creative effect, in this case making the sky and colours stand out while letting the rest of the frame fall to darkness.
