Would love to hear tips and links to research. I've seen a few studies on daytime running lights reducing accidents in motorcycles, cars, and bicycles. It is interesting how they do the studies -- because the argument is always that having lights on may make someone drive more safely.
It's taken time but I have convinced a few cyclists to spend more money on ultra-bright front lights (daytime, 1000+ lumen flashing) and rear lights (300 lumen) and have them on at all hours of the day.
One argument that works is showing before and after photos of a cyclist in shadows (whether a building or tree shadow). There is a clear difference in visibility, and it's obvious you can't plan in advance at high speeds if you hit shadows and don't know what's behind you. It is too much cognitive load, so worth spending the extra money up front to feel safer should you happen to bike into shadows.
It can feel wimpy to have lights on all the time, so what have you found works for convincing people you love to get that added bump in safety? This can elicit strong emotions so that's why I'm trying to figure out softer ways to have this conversation and hopefully we might save some lives!
In general, it's not hard for attentive motorists to see cyclists during daylight hours and good visibility. Even if cyclists are in shadows or there is a low hanging sun. Bright colors and lights are just ways to try to draw the attention of motorists who are not tuned into their surroundings. Cyclists will arrive at different places in their efforts to enhance their conspicuity to try to capture the attention of distracted drivers, and it's not my place to convince them to do otherwise. Every day across this country people have accidents with tractor trailer rigs when they "didn't see them." I am not going to try to convince truck drivers to hang a disco ball on the front of their rigs or paint them neon colors.