Cosmologists all agree that the present state of the cosmic background radiation is in the microwave region of the electromagnetic spectrum. This corresponds to a blackbody radiation temperature of about 3K. Since our naked eyes are sensitive only to the visible spectrum, the night sky does appear dark to all of us. But if we assume a continuous decreasing change of EM energy since the Big Bang, the night sky did appeared in the visible region in the distant past. I am looking for a simple formula that will calculate the epochs where the night sky appears as red, orange, yellow, green, blue, indigo, and violet. Anybody has any idea?