What is the meaning of "va'Omar Ach Choshech Yeshufeni"?
Rashi: If I said that I will be hidden in darkness, and You will not see me. "Yeshufeni" is based on Neshef 1 (evening).
Radak: If I said that darkness will hide me, that You will not see me, my deeds and my thoughts...
Radak: Neshef and Shuf are two roots with similar meanings. the same applies to Namal and Mul, Nafatz and Putz.
Why does it say "v'Laylah Ohr Ba'adeni"?
Rashi: [If I said that] night will darken in front of me. Here, "Ohr" is an expression of darkness, like "Yafitz Anan Oro" (Iyov 37:11), "va'Ya'er Es ha'Laylah" (Shemos 14:20).
Radak: Night, which is dark, enlightened for me, and You saw me at night like during the day.
Malbim: I thought that the place of darkness will darken [cover me] during the day, and all the more so at night (but it did not).