If it is a Black and White Ball the invitation will say so.
If gentlemen are requested to wear scarlet- the etiquette is - scarlet and white tie for gentlemen with colors, white tie and tails for gentlemen without colors, ball gown for ladies (no red)
Black tie - tuxedo for gentlemen without colors, evening gowns for ladies. That invitation will also indicate that a black tie is permitted by saying, “Scarlet if convenient”. This is a less formal than a truly formal event - (but still formal- business suits or knee length dresses would not be appropriate)
That’s the etiquette - but you don’t usually see women in ball gowns that much anymore - not real ball gowns. Evening gowns of all kinds seem to be normal now.
Usually what you see at a semi-formal ball is gentlemen with colors wearing scarlet and white tie (formal), gentlemen without colors wearing a tuxedo (black tie), and women wearing evening gowns or cocktail dresses.
If the ball has a theme, or if the ball is truly less formal - then the attire etiquette isn’t a big deal, not that anyone follows it much anymore anyway. I’m just telling you what the etiquette is, because few people know such stuff anymore (just like what glasses to use or which fork to use first in a formal table setting) The diamonds and denim theme sounds like it was a lot of fun - I bet that was a helluva party.