Ružičasta haljina joj lijepo stoji, dok je ljubičasta još ljepša.

Yes, the basic meaning of stajati / stoji is to stand, but in sentences about clothes it often means to suit someone, to look good on someone, or to fit someone well in appearance.

So:

• Haljina joj lijepo stoji = The dress looks nice on her / suits her well

This is a very common Croatian use. It is about how the clothing looks on the person, not literally standing.

Joj means to her and is the dative form of the pronoun ona.

In this expression, Croatian uses the pattern:

• nešto nekome lijepo stoji = something looks good on someone

So:

• Ružičasta haljina joj lijepo stoji = The pink dress suits her nicely

Literally, it is something like:

• The pink dress stands nicely to her

but in natural English that becomes looks good on her or suits her.

Because joj is a short pronoun form, it is also a clitic, which affects word order.

Because lijepo is an adverb, not an adjective here.

It modifies the verb stoji:

• lijepo stoji = looks nice / suits nicely

Compare:

• Haljina je lijepa. = The dress is beautiful.
  Here lijepa is an adjective describing haljina.

• Haljina joj lijepo stoji. = The dress looks nice on her.
  Here lijepo is an adverb describing how it looks on her.

This is a very common distinction in Croatian:

• adjective: lijep / lijepa / lijepo
• adverb: lijepo

The adverb often has the same form as the neuter adjective.

Here dok means while, whereas, or while on the other hand. It introduces a contrast between the two parts of the sentence.

So:

• Ružičasta haljina joj lijepo stoji, dok je ljubičasta još ljepša.

means something like:

• The pink dress looks nice on her, while the purple one is even prettier.
• The pink dress suits her well, whereas the purple one is even prettier.

Important: dok can also mean while in a time sense in other contexts, but here it is mainly contrastive.

Yes, that is completely normal.

In the second clause, haljina is omitted because it is already understood from the first clause. Croatian often allows an adjective to stand on its own when the noun is obvious.

So:

• ljubičasta here really means ljubičasta haljina
• in English, we often translate that as the purple one

This is similar to English:

• I like the red shirt, but the blue is better.
• more naturally: the blue one is better

So in this sentence:

• dok je ljubičasta još ljepša = while the purple one is even prettier

Ljepša is the comparative form of lijepa:

• lijepa = beautiful / pretty
• ljepša = more beautiful / prettier

It is feminine singular because it refers to the implied noun haljina, which is feminine singular.

So the structure is essentially:

• ljubičasta haljina je još ljepša = the purple dress is even prettier

Even though haljina is omitted, the adjective still agrees with it in gender and number.

Here još means even.

So:

• ljepša = prettier
• još ljepša = even prettier

This is a very common Croatian pattern:

• bolji = better

• još bolji = even better

• veći = bigger
• još veći = even bigger

In this sentence, još strengthens the comparison.

The short pronoun joj is a clitic, and Croatian clitics usually go in the second position in the clause.

Here the first unit is:

• Ružičasta haljina

Then the clitic comes:

• joj

So:

• Ružičasta haljina joj lijepo stoji

is the natural order.

Croatian clitic placement can be tricky for learners, because it is not exactly the same as “second word”; it is more like second position in the clause after the first phrase/unit.

That is why joj appears where it does.

They are all in the nominative feminine singular.

• Ružičasta haljina is the subject of the first clause.
• ljubičasta stands for ljubičasta haljina, so it is also nominative feminine singular.
• ljepša agrees with that same implied feminine singular noun.

So the agreement is:

• haljina — feminine singular
• ružičasta — feminine singular
• ljubičasta — feminine singular
• ljepša — feminine singular comparative

This is why the endings are the way they are.

Usually it is mainly about appearance: how the dress looks on her, how flattering it is, whether it suits her.

So:

• Haljina joj lijepo stoji usually means The dress looks good on her

It can sometimes imply that the cut or shape suits her body well, but it is not specifically about whether the dress is physically the correct size.

If you want to emphasize physical fit more directly, Croatian might use other wording too, depending on context.

Yes. That is a very natural translation.

A few good English versions are:

• The pink dress looks nice on her, while the purple one is even prettier.
• The pink dress suits her well, but the purple one is even nicer.
• The pink dress looks good on her, whereas the purple one is even prettier.

The exact English choice depends on whether you want to emphasize:

• how it looks on her → looks good on her / suits her
• beauty of the dress itself → prettier / nicer

Croatian allows a little overlap here, so several translations can work.