Don't pay attention to the two previous responses. Blue Iridium has a 30% light transmission rate, which puts it on par with other sunny day lenses, but at 30% it's not quite a one-trick pony and you can still get away with it on medium light days. (All of the light transmission ratings are at Oakley's website.)

Black Iridium is closer usually best for sunny days, as its light transmission rate is 18%.

Despite all of this, don't automatically equate "day riding" with "dark lenses." I usually only ride days, but the Pacific Northwest is usually overcast, so my HI Yellows (81% light transmission) see the most use.