I suspect if there’s ever a practical way to differentiate otherwise similar looking characters when out of costume, it’s going to be a challenge if one’s used to a template though others do bother to change and deviate. (I swear they may bother to learn more.) Let’s take Cassandra Sandsmark and Stephanie Brown. They are two different characters with the former being quite obviously a nerdy fangirl especially early on.
(Which I find it ironic as Cassie should have an advantage over Donna Troy, being much more accessible by being a Wonder Woman fanatic.)
They might be some attempts to differentiate the two but that would be putting a lot more thought on their personalities, far more than what canonical/official media provide. It’s like what would happen if Cassie likes dressing in florals, pastels and bright reds (that’s even there in canon) but Stephanie tends to err closer to goth-punk clothing and purple (also canon).
Especially when out of costume, this is enough to give an idea of what their actual fashion sensibilities are like. Cassie Sandsmark’s never a masked character and needn’t one at all. Stephanie’s no stranger to wearing masks, one might wonder if she prefers to be guarded from people she doesn’t trust (if she’s stated to have a temper and be mean to criminals, one wonders if she wants to intimidate people).
There’s even something like fashion psychology which analyses and understands how and why people dress the way they do. Especially with regards to the big five personality traits of extraversion/introversion, calmness/neuroticism, agreeableness/disagreeableness and conscientiousness/stubbornness. Keep in mind that this may not always be the case for others.
Whilst tendency to correlate dark clothing with neuroticism only makes sense among Goth circles, among some sports teams and religious communities this should indicate conscientiousness and agreeableness. Ad infinitum, ad nauseum. If applied to Cassie and Steph or even other women, this should enable character designers to delve more into their personalities even if they want to put in their ideas of them.
Like I said, Cassie Sandsmark’s never a masked character to begin with. So she should be easily seen wearing florals, pastels and bright reds whenever out of costume. Steph on the other hand is no stranger to wearing a mask and in her earliest outing as Spoiler, her hair’s invisible. If she’s also shown to be sarcastic and grouchy, even to her own dad, she should also score lower on agreeableness.
The fact that if she’s said to do things against what others like Batman expects her to, then it shouldn’t be a stretch for her to score lower on conscientiousness and agreeableness, albeit somewhat more than Cassie does. This does turn character design on its head, if because it necessitates people to get into the characters themselves.
Especially in what they want to and will to wear.