Ohhh, this is such a good question
Andi's childhood step 1 fashion style is 'chosen by mom', Andi as a 10 years old absolutely doesn't have a clear fashion sense preference as her mind only cares about play, school, and the new people she's meeting in Golden Grove. so I like to think that Opal would be the one picking her outfits lol
As she gets older she begins wearing feminine clothes, experimenting with cute and elegant style, before eventually opting as an adult for elegant/ semi-professional wear, blazers, shirts, ties, elegant skirts, high heels as well as incorporating her favorite colors, blue and purple into her fashion style.
Though she'd avoid dressing in pink shades because she wouldn't want to look too 'bubblegum-themed', given her pink hair.