HI yellows are the absolute best for flat light days like you're explaining (white out or foggy days). They're not very versatile though, IMO. On a variable light day, they tend to amplify the light too much, like when the sun might poke through for a little bit. On days like those, the HI persimmon would be perfect. I'll actually be in the market for some HI persimmon lenses next year. Right now I'm rocking a set of fire lenses and HI yellows, two lenses on each end of the spectrum with nothing in between, haha. I'd recommend getting the HI yellows since you have some good mid spectrum lenses already.