Shall I compare thee to a Summer's day?
Thou art more lovely and more temperate:
Rough winds do shake the darling buds of May,
And Summer's lease hath all too short a date:
Sometime too hot the eye of heaven shines,
And often is his gold complexion dimm'd;
And every fair from fair sometime declines
By chance or nature's changing course untrimm'd:
But thy eternal Summer shall not fade,
The  personified  sun is often “dimmed”, meaning that clouds or bad weather can block the beauty. The noun “complexion” applies to the sun, but it also applies to the young man.
Sonnet 18