I deliver mail for my job from salt lake city to denver and back again. Same route weekly, but this week was different. The sky was partly cloudy, and the temperature was about 36 degrees. I was headed east on I80 just north of elk mountain Wyoming about 515am. When I looked southeast towards elk mountain, I notice a dark cloud over it. The second time I looked, I seen 3 to 4 purple or blue flashes that flashed within the cloud in a sequence that lasted about a second which illuminated the cloud to a greenish color. I've never seen anything like this before. The event startled me so much I almost lost control of my vehicle.