Somewhat dark - Rather, somewhat faint. Compare Leviticus 13:6.
Matthew Henry
Concise Bible Commentary
The garment suspected to be tainted with leprosy was not to be burned immediately. If, upon search, it was found that there was a leprous spot, it must be burned, or at least that part of it. If it proved to be free, it must be washed, and then might be used. This also sets forth the great evil there is in sin. It not only defiles the sinner's conscience, but it brings a stain upon all he has and all that he does. And those who make their clothes servants to their pride and lust, may see them thereby tainted with leprosy. But the robes of righteousness never fret, nor are moth-eaten.