If you dream of red sky, you could be seeing dream imagery of a sunrise or sunset. Its more likely to be of a sunset. The meaning of dreaming of a sunset can be interpreted from your present circumstances in life. Are you feeling happy or sad? If you are feeling sad, then this sunset sky imagery could be a reflection of your negative emotion.
Red is a warm color. Dream imagery of a red sky can be interpreted as an angry sky. Why is the sky angry? Again, the interpretation depends on your personal experiences and situation in life.