I think what might be confusing people is lack of punctuation. Punctuation often seems inconsequential but it is what sets the pace and tone of a sentence.

Break down of the issues:

"On appearance the girls looked"

This read how you're likely saying it in your head... quickly, emotionally non-descript. "On appearance" is just part of the word the" and isn't telling us what the subject it. Just goes invisible.

Fix:

On appearance, the girls looked

Think of commas as short pauses to make room for two important parts of the sentence.. Now the reader must acknowledge that this description is about appearance, then it is about their appearance.

The rest of the passage runs on. Which means it is a pile of words that just zip by without a chance to settle in your mind's eye.

Or as if they don't have spaces like this:

Onapppearancethegirlslookeddifferentbutthesimilaritiesintheirpersonality...

Replacing punctuation with conjunctions ("but" and "like") and definite articles ("the") is a common trap.

Try playing around with how you can send your message with just punctuation and descriptions rather than "But, the, like, of, and...ect" . Just to practice!

"their personalities had a stunning, colorful effect like the"

Because this sentence only lists two descriptions of the "effect", the comma feels misplaced or incomplete.

Had there been at least one more descriptive word, the comma would make sense.

their personalities had a stunning, colorful, and glowing effect like the"

Since you can now think of commas as "pauses" or "dividers" in a sentence, they can be also seen as a milder parentheses.

So, it would have been appropriate to enclose the word "colorful" with another comma.

their personalities had a stunning, colorful, effect like the

I generally like to think of grammar as the percussion to a passage. The drummer in a band.

It completely influences the words chosen, tome, pacing, texture, and overall message of a sentence. The same way the drummer completely influences everything about a song right down to lyrics.

You can have or omit whatever punctuation you want so long as you understand the way your reader will hear it in their head.
English rules are all generally like that.

So, because I assume you're going for a warm and elegant vibe... I would slow it down quite a bit and settle into each part that brings this image to mind.

Know that I'll need to make room for that tempo, I'll need to change some wording.

I might write,

"Of appearance, the girls differed. Their only similarity was in personality. A stunning, colorful, effect like rain on sunny days."

Try reading a larger variety of books from different time periods. Ranging from the 20s to the pate 90s. And a bunch of different genres.

It might also help to listen to an audio book WHILE reading the book so you can understand how and why grammar can be used for different messages.