On the Torah׳s wording וראהו הכהן ביום השביעי the Sifra interprets ביום literally, by day and not at night.
Too, viewing isn’t done indoors nor on a cloudy day.
The kohen’s POV
There are a few rules any kohen viewing nega’im must be in the know about. Of them, timing -and more importantly proper lighting, is essential for the kohen to be impressed with a kosher view of the affected person, garment or property.
Out of concern that the shade of the nega be correctly displayed, good lighting is required, not too bright and Not too dark, viewing isn’t allowed to be done indoors and is done during between 3rd and 5th hour of the morning (שעות זמניות) or between the 7th and 9th hour of the afternoon.
Rabbi yehuda says the 4th and 5th hours of the morning or the 8th or 9th hour of the afternoon.
Rabbi Yossi says the 4 5 9. 10
Sifra tazria min. Ch. 60
Nega’im 2 2