vr28: darker than persimmon. It is a fairly versitile lens, decent for cloudy days and such. since it has less light transmission, it is better for brighter conditions than persimmon.
persimmon: all-around lens, best for lower light, and even night, not too great for sun though
HI persimmon: has the blue HI glare. increases contrast a tiny winy bit. It allows pretty much the exact same amount of light as persimmon, so same conditions as persimmon
iridium (any "mirror" lens if you will): sunny or bright. the iridium coating reduces glare. really fucking hard to see in fog and low light, and forget about using it at night
black iridium allows the least light, then gold iridium, then blue, then fire