Metto un rossetto rosso per la serata.

Metto is the 1st person singular present tense of mettere, so it means I put / I put on / I apply.

In this sentence, metto is being used in the sense of putting on lipstick or applying lipstick.

• mettere = to put, to place, to put on
• metto = I put / I put on

With makeup, Italian often uses mettere where English might say put on or apply.

Italian often drops subject pronouns when the verb already makes the subject clear.

Here, metto clearly means I put / I am putting on, so io is unnecessary.

• Metto un rossetto rosso... = I put on a red lipstick...
• Io metto un rossetto rosso... is possible, but it adds emphasis, like I am putting on a red lipstick.

This is very normal in Italian.

Italian usually needs an article where English sometimes does not.

So:

• un rossetto = a lipstick
• not normally just rossetto on its own in this kind of sentence

The article un is the masculine singular indefinite article, because rossetto is a masculine singular noun.

Because rossetto is grammatically a masculine noun in Italian.

That is simply part of the word’s grammar:

• il rossetto
• un rossetto

Its gender affects the article and the adjective:

• un rossetto rosso

Both un and rosso are masculine singular to match rossetto.

In Italian, color adjectives usually come after the noun.

So:

• un rossetto rosso = a red lipstick

This is the normal word order in Italian:

• noun + adjective

English usually does the opposite:

• adjective + noun

So compare:

• English: a red lipstick
• Italian: un rossetto rosso

They are related in sound, but they are different words with different meanings:

• rossetto = lipstick
• rosso = red

The similarity is not a mistake. It may sound repetitive to an English speaker, but it is completely natural in Italian.

So un rossetto rosso simply means a red lipstick.

Yes, you may hear that too.

• Metto un rossetto rosso = I put on / apply a red lipstick
• Mi metto un rossetto rosso = I put on a red lipstick on myself

The version with mi can sound a little more personal or closer to I’m putting it on myself, which is natural because lipstick is something you apply to yourself.

In everyday Italian, both kinds of wording can appear, though mettersi is especially common with clothing and personal appearance.

Most directly, it means I put on / I apply.

Depending on context, present tense in Italian can sometimes sound natural in English as:

• I put on a red lipstick for the evening
• I’m putting on a red lipstick for the evening

If the focus is on the action of application, apply is a good translation. If the focus is on getting ready, put on works very well.

To mean I am wearing red lipstick, Italian would more naturally use something like:

• porto un rossetto rosso
• or a different phrasing depending on context

So in this sentence, the main idea is the act of putting it on.

Per la serata means something like:

• for the evening
• for tonight
• for the evening event / occasion

It suggests that the red lipstick is being chosen because of the evening plans or for an evening occasion.

So the idea is not just any time of day, but specifically an evening context.

Italian often uses the definite article in places where English might not.

So:

• per la serata = for the evening

The article la makes the phrase sound natural and complete in Italian.

You will often see this kind of structure:

• per la cena = for dinner
• per la festa = for the party
• per la serata = for the evening

Not exactly.

• sera usually means evening in a general time-of-day sense.
• serata often suggests the evening as an occasion, the evening event, or the night out.

So per la serata can sound a bit more like:

• for the evening
• for the night
• for the evening out / evening occasion

That makes it a good choice in a sentence about makeup or getting ready.

You might hear related expressions, but per la serata is very natural here because it suggests an evening occasion.

• per sera can mean for this evening / by this evening in some contexts
• per la sera sounds more like for the evening time
• per la serata sounds more like for the evening event / tonight’s look

So for makeup, style, or preparation, per la serata fits especially well.

Rossetto is the lipstick product.

So:

• un rossetto = a lipstick
• un rossetto rosso = a red lipstick

Depending on context, English may translate that as either:

• a red lipstick
• red lipstick

But in Italian, the noun refers to the lipstick item/product, and rosso specifies the color.

Yes. Italian often uses the present tense for actions connected to near-future plans or current preparation.

So Metto un rossetto rosso per la serata can mean something like:

• I’m putting on a red lipstick for the evening
• I put on a red lipstick for the evening

It can describe what the speaker is doing now as part of getting ready for something that will happen later.

This is very normal in Italian.

Not really in this case.

Indossare is mainly used for wearing clothing or accessories, such as:

• indosso un vestito
• indosso una giacca

For lipstick or makeup, mettere is much more natural:

• metto il rossetto
• metto un rossetto rosso

So metto is the right choice here.

Yes, it is understandable and natural.

A native speaker might also say things like:

• Metto un rossetto rosso per stasera.
• Mi metto un rossetto rosso per la serata.
• Per la serata metto un rossetto rosso.

But the original sentence is perfectly fine and natural for expressing the idea.