In keeping with the subject in the opening post; If you are going to use a rear lamp during the day it has to be extremely bright and at a very "attention getting" flash setting. Otherwise, in full daylight I really don't think it would be helping.

I agree 100% about wearing a the bright daytime color jersey or jacket. The only color I wear on the road is the extremely bright daytime yellow. IMO it is the best color to wear but the neon green and fluorescent orange are pretty good as well.

I have at times used a rear lamp in the daytime ( rear ) but only when on really busy roads with fast traffic. I will however turn my rear lights on once the sun is lower in the sky. My daytime rear lamp is a torch I have mounted to my seat. It has an amber XP-G2 led and uses a reflector. The flash pattern is a triple flash/pause. I like it, very attention getting but it might only be about 150 lumen ( custom driver/over-driven ). I've considered buying the DiNotte quad amber to replace it but I want to be sure the DiNotte will hands down be at least twice as visible ( "At Distance" ) before I lay down the big money. ( Sadly if I want to do that I'll have to buy one and then decide if I want to return it ).

Now someone might wonder why I would want an "amber" lamp for the rear....well, to be to the point, I want to stand out, be different. I know someone will say that the red LED's are brighter...I know that. If you've never seen a good extremely bright amber lamp you wouldn't know just how attention getting they can be though.

I've seen good rear ( red ) bike lights in use during the day. The good ones use a very bright flash mode. Last year I came up on one guy using one. I could see his lamp from almost a half mile away. ( before I even knew it was a cyclist ). It was funny too because he was riding with another guy who was using just a cheap rear led, his I could barely see and wouldn't have even noticed if it wasn't for the other guy with the bright one. The difference between the two was like the difference between night and day. I don't know what lamp the guy was using but when I caught up to him I wound down my window and let him know how well his lamp was working.