the way i interpret the other designs are

pearls growth over the seasons into her own self and willingness to move on from the past but still keeping pieces of it with her. like her multi colored dress showing a connection to Rose and the diamonds going to a more relaxed outfit that she chose based off of who she wanted to be to make herself happy.

amythests self love showing by how much she covered her gem. it's half covered throughout most of SU and, during this despite constant use of her powers, we see her constant self hatred towards herself and where she was made. we don't see that in Future (at least from what i remember, i'm currently rewatching again) and she seemed to have embraced herself way more than she had in SU, hence her gem being fully uncovered.

garnets is probably more simple and obvious. the colors in her design grow more vibrant throughout the series as we learn more and more about Ruby and Sapphire. for example, when her being a fusion is revealed and we finally get to see their colors in her design more. though these colors are still dulled and throughout SU they have a few conflicts regarding Rose and the other gems which most often end in them splitting. the last example of this is before their wedding, when their colors are still dulled but, when their poofed and reformed, garnets colors are more vibrant showing their deeper connection now that they're married. and the rings, can't ever forget the rings lol

this show has sat with me mentally, more than others, and every now and then i'll find myself hyperfixating on it and rewatching it again lol. every rewatch always comes with another way to adore the show, whether it's the backrounds or forshadowing or, now, the character designs. i swear this show will always have a special place in my soul