I think that the biggest reason that your fashionable older lady said that was becasue back in her day the "rule". Fashion was so much more black & white back then. (pun intended) They would never have worn black leather in summer, maybe a straw or bamboo summer thing in black, never white after Labor day, now look at al of the beautiful white winter fashhions are out there. (Beautiful white suits, the new gorgeous white Channel bags and watches, etc.) Also one would never hve worn open toe shoes in winter.
I think it is just what the fashion rules were in their day. For example, my grandma always wore a dress, pearls and low heels everyday even around the house, would never have worn jeans or sweats or even sit around the house without all her make up on even if she wasn't leaving the house or anyone was coming over, just becasue that was the way that she was raised.
Now for me personally, I wouldn't carry black leather in summer, but i love love love all of the fun bright colors and actually have to make myself buy and carry a black bag in the winter. I wouldn't do white in winter either, but then agian I personally won't do white everyday bag at all because I would get it dirty or scratched and it would freak me out.