Aujourd’hui, elle a choisi un rouge à lèvres discret, mais elle n’a presque pas mis de mascara.

This sentence uses the passé composé, the most common French tense for talking about completed actions in the past.

• elle a choisi = she chose / she has chosen
• elle n’a presque pas mis = she hardly put on / she barely wore

The passé composé is formed with:

• a present-tense auxiliary verb, usually avoir
• plus a past participle

So:

• a choisi = has chosen
• a mis = has put on

In everyday French, this is usually the normal way to say what someone did.

Aujourd’hui means today.

Historically, it comes from older words meaning something like on the day of today, which is why it looks unusual. The apostrophe is just part of the fixed spelling: aujourd’hui.

You should simply learn it as one common word meaning today.

Yes, lèvres means lips, but un rouge à lèvres is a fixed expression meaning lipstick.

Literally, it is something like:

• rouge = red
• à lèvres = for lips

Even though lèvres is plural, the whole expression is a singular noun:

• un rouge à lèvres = a lipstick
• des rouges à lèvres = lipsticks

So you should treat rouge à lèvres as one vocabulary item.

Because discret agrees with rouge à lèvres, which is a masculine singular noun.

• un rouge à lèvres discret = a discreet/subtle lipstick

Even though lèvres is feminine plural inside the expression, the main noun is rouge à lèvres, and that whole noun phrase is masculine singular.

So the adjective must be:

• discret for masculine singular not
• discrète for feminine singular

Here, discret means subtle, not too noticeable, or understated.

With makeup, un rouge à lèvres discret suggests:

• a subtle lipstick
• a natural-looking shade
• something not too bold

It is related to English discreet, but in this context subtle or understated is usually the best translation.

Presque pas means almost not or, more naturally in English, hardly / barely.

So:

• elle n’a presque pas mis de mascara means
• she hardly put on any mascara
• she barely wore any mascara

In French negation, ne and the second negative element go around the conjugated verb:

• elle a mis = she put on
• elle n’a pas mis = she did not put on
• elle n’a presque pas mis = she hardly put on

So presque pas softens the negation: not zero, but very little.

After most negations in French, du / de la / des usually change to de (or d’ before a vowel sound).

Compare:

• Elle a mis du mascara. = She put on some mascara.
• Elle n’a pas mis de mascara. = She didn’t put on any mascara.
• Elle n’a presque pas mis de mascara. = She hardly put on any mascara.

So de mascara is expected here because the phrase is negative.

Mis is the past participle of mettre, which usually means to put.

But in French, mettre is also commonly used for:

• putting on clothes
• applying makeup
• wearing certain items

So:

• mettre du mascara = to put on / apply mascara
• mettre du rouge à lèvres = to put on lipstick

That is why elle a mis du mascara sounds natural in French.

This is called elision.

When ne comes before a word starting with a vowel or silent h, the e drops and becomes an apostrophe:

• ne a → n’a
• ne est → n’est
• ne ont → n’ont

So:

• elle n’a presque pas mis is just the normal spelling and pronunciation.

Mais means but.

It connects two contrasting ideas:

• elle a choisi un rouge à lèvres discret
• mais elle n’a presque pas mis de mascara

So the sentence contrasts:

• she did choose a subtle lipstick, but
• she hardly wore any mascara

It works just like but in English.

Yes. Grammatically, the French passé composé often corresponds to either:

• the simple past in English: she chose, she hardly put on or
• the present perfect: she has chosen, she has hardly put on

In natural English, for a sentence like this, the simple past is usually more idiomatic:

• Today, she chose a subtle lipstick, but she hardly put on any mascara.

But depending on context, the present perfect can sometimes also work.