This line is yet another comparison to the colors of spring turning from gold to green, and the fall of Adam and Eve, but this time, dawn is going down to day.
Think of refreshing, golden hues of sunrise turning into the scathing heat of a summer day.
The use of the word "down" in this line makes us think… isn't the sun supposed to rise in the morning? Wouldn't dawn rise up to day? Well, yes, literally, the sun does come up. But the speaker prefers dawn. Day is at a lower level because, possibly, it's lost the freshness and color of the dawn.