Here are some simple examples:

"röd sko - red shoe" singular n-Word

"Röda skor - red shoes" plural n-Word

"Rött hus - red house" singular t-Word

"Röda hus - red houses" plural t-Word

Then there are the comparative ones:

"Rödare - more red"

"Rödast - most red"

Correct me if i am wrong but this goes for all adjectives, not only colours.

In case you haven't picked up on this yet, this happens all over the language - words that apply to nouns have to agree with the noun's gender/plural status. So for every (most?) word that applies to a noun, you'll get three variations that fit a general -t, -n,-a pattern (for ett, en and plural nouns respectively). Think of them as different ways of saying the same basic word

So for adjectives like colours you'll have these versions, and you use the one that agrees with the noun you're applying it to. For possessives like 'my', you have mitt, min and mina. For things like 'I have no noun', 'no' is inget, ingen or inga. See the general pattern? It's similar to languages like Spanish, where everything has to match the noun in gender and number. el perro blanco es mío, las gatas negras son mías (os and as and plurals, see?)

(I'm still new to Swedish so sorry for any glaring mistakes, but I hope the general idea helps people see the pattern and makes things easier!)

My takeaway as a learner: "har på sig" is a phrase that translates as a whole to "wears" or "is wearing" in English.

"The girl is wearing her red shoes" would be "Flickan har på sig sina röda skor"

Maybe 'har på sig' directly goes to something like has on her

Yes, exactly.

You mean the röda, röt, rött ? It's because it changes depending on the word's gender and quantity (one or more)

Yes, absolutely.

If you say "the girls", they're in the plural so you need "are wearing". But the Swedish flickan is the singular definite, so it has to be "the girl" - a single one.

That is correct.