Coral is far more red than her lips' red
I see the repeated use of the colors red and white holding significance in the poem. There is a constant comparison of there being a lack of vibrancy in this mistress, with the color red being a descriptive term. If I am to take the term "mistress" under the definition of a woman having an extramarital affair, this could be seen as the speaker seeing all of the features he may have taken for granted at the time. The rose color of her cheeks and lips, the angelic white glow, all are muddied and faded as this is not the woman he truly loved.