So, you never humoured your grandmother's fashion pet peeves on her turf? I never knew my grandmothers, but my mother (like the Queen's) had plenty of opinions, and if you didn't want to hear an earful, you just avoided giving her the ammunition. Once I got past the teen rebellion stage I found it much less stressful to just go along to get along on clothing choices; fashion at my Mom's house just wasn't the hill I wanted to die on.
Members of the working royal family are being compensated by the Crown when they make public appearances, so you might try to consider it an office dress code, because that is what it is. She feels it's about dressing professionally and respectfully, and being careful about unintentionally offending the people who come to see public appearances. Yes, part of it is her personal preference (the wedge shoe rule comes to mind), and some of her perceptions about what is "decent" hark back to the 1950's, but you also have to realize that her own mother was much more draconian about "the rules", and the present Queen humoured the Queen Mother's choices long past the point when most women would. She has loosened the rules over the years; for instance, she no longer insists on gloves or girdles for anyone other than herself. I guarantee you she finds the idea of bare legs in public distasteful because like all Englishwomen of her generation, she was raised to think that bare legs in public were louche. (Remember all those stories about women painting fake seams down their legs during WW2 because stockings were scarce? They were ashamed of being barelegged because they were taught that a grown woman going out that way was going out half-dressed, so they used makeup and kohl pencil to fake it.) The "tights" rule doesn't really require tights, as stockings are fine as well, and coloured tights are OK, too, but the rest of the list is really preference built up over 8 decades of endless public appearances. She knows what styles have high potential to accidentally embarrass the wearer. (For instance, while she doesn't care for dark nail polish, she also has the experience to know that dark polish makes a broken nail much more glaringly obvious when you're shaking 100 hands.) The prejudice against black in the daytime is partly a holdover from the War, when it was considered sad, but also is practical; black ages badly with frequent dry cleaning, and it shows lint.