I live in a house with white siding -- at least it looks white most of the year. However, when the snow falls in the winter, all of a sudden my house looks dingy and yellow. What may look "clean" when we compare ourselves to other sinners takes on a whole different cast when seen next to the perfect holiness of God.
The way to see the Truth about sin is to see it in the light of who God is. When we gaze upon the brilliance of His untarnished holiness we become acutely aware of the hideousness of our sin.
I think women are especially prone to compare ourselves to other women. It's much easier to give ourselves a proverbial pat on the back when we are comparing ourselves with other sinners. But that's not our standard. It's only when we compare ourselves to the "brilliance of His untarnished holiness" (I LOVE that phrase!) that we can truly see our dinginess. Our filthiness. Our unworthiness.