Questa camicetta viola ti sta bene.

Because the verb agrees with questa camicetta viola, not with you.

In this sentence, the subject is questa camicetta viola = this purple blouse, so the verb is third-person singular: sta.

Ti means to you / on you, so you are not the subject.
That is why Italian says ti sta bene, not ti stai bene.

Literally, it is something like it sits well on you or it is well on you.

In natural English, with clothes, it usually means:

• it looks good on you
• it suits you
• sometimes it fits you well

Italian often uses stare bene for how clothes, colors, or styles look on someone.

Ti is the unstressed pronoun for to you in the singular informal form.

So:

• mi sta bene = it suits me
• ti sta bene = it suits you
• gli sta bene = it suits him
• le sta bene = it suits her

Italian normally uses this short pronoun before the verb instead of saying a te.

You can say a te too, but usually only for emphasis:

• A te questa camicetta viola sta bene. = This purple blouse looks good on you, at least on you.

Because camicetta is a feminine singular noun.

The demonstrative adjective must agree with the noun:

• questo for masculine singular
• questa for feminine singular
• questi for masculine plural
• queste for feminine plural

So:

• questa camicetta = this blouse
• questo maglione = this sweater
• queste scarpe = these shoes

Because questa already works as a determiner.

In Italian, a demonstrative like questo / questa normally replaces the article:

• questa camicetta = this blouse
• la camicetta = the blouse

So you do not say la questa camicetta in normal Italian.

Because color adjectives usually come after the noun in Italian.

So:

• camicetta viola = purple blouse
• maglia rossa = red shirt
• scarpe nere = black shoes

Putting viola before the noun would sound unnatural in ordinary Italian.
For a learner, the safest rule is: noun + color.

Because viola is usually an invariable color adjective.

That means it stays the same with masculine, feminine, singular, and plural:

• una camicetta viola
• un cappello viola
• due camicette viola
• due cappelli viola

Not all color adjectives behave this way. Some do change:

• nero / nera / neri / nere
• rosso / rossa / rossi / rosse

But viola, like rosa and blu, normally stays the same.

Not exactly.

Camicetta usually means blouse, typically a women’s blouse.
Camicia usually means shirt.

Historically, camicetta is related to camicia, but in modern Italian it is a normal vocabulary word, not just a transparent little form you need to analyze every time.

So in this sentence, camicetta is best understood simply as blouse.

Yes, Italian can move parts of the sentence for emphasis.

The neutral order here is:

• Questa camicetta viola ti sta bene.

But you could also hear:

• Ti sta bene questa camicetta viola.

That version puts more focus on ti sta bene or sounds a little more conversational.

However, camicetta viola should normally stay together, with the color after the noun.

Often it can mean either, depending on context.

With clothing, ti sta bene commonly suggests:

• the garment looks good on you
• the style or color suits you
• sometimes the fit is good too

If the context is about appearance, English will often translate it as it looks good on you or it suits you.

If the context is about size and fit, English might use it fits you well.

So the Italian phrase is a little broader than any one single English translation.

You change the pronoun, and if the clothing item is plural, the verb changes too.

Examples:

• Questa camicetta viola mi sta bene. = This purple blouse looks good on me.
• Questa camicetta viola gli sta bene. = This purple blouse looks good on him.
• Questa camicetta viola le sta bene. = This purple blouse looks good on her.

With a plural subject:

• Questi pantaloni ti stanno bene. = These pants look good on you.

Notice:

• singular subject: sta
• plural subject: stanno