Je ne porte pas de T-shirt rose au travail, et Marie non plus ; elle préfère des couleurs plus discrètes.

Because French usually makes a statement negative with ne ... pas around the verb.

• Je porte = I wear / I am wearing
• Je ne porte pas = I do not wear / I am not wearing

In everyday spoken French, people often drop ne, but in standard written French you normally keep both parts:

• Je ne porte pas...

After a negation, French often changes un / une / des to de (or d’ before a vowel sound).

So:

• Je porte un T-shirt rose. = I wear a pink T-shirt.
• Je ne porte pas de T-shirt rose. = I do not wear a pink T-shirt.

This is a very common rule after ne ... pas.

In French, borrowed words still get a grammatical gender. T-shirt is masculine, so you say:

• un T-shirt
• de T-shirt after negation

That is also why the adjective is singular masculine here:

• rose with T-shirt

Most French color adjectives come after the noun.

So:

• un T-shirt rose
• des chaussures noires
• une robe bleue

French and English often place adjectives differently, so this is a very common thing to notice.

It actually already matches the noun, but rose is one of those adjectives whose masculine singular and feminine singular forms look the same.

Here, T-shirt is masculine singular, so:

• rose = masculine singular

If the noun were plural, you would add -s:

• des T-shirts roses

Au is a contraction of à + le.

• à le travail becomes au travail

So au travail literally means at the workplace / at work.

This kind of contraction is very common:

• à + le = au
• à + les = aux
• de + le = du
• de + les = des

Because the first statement is negative, and French uses non plus to mean neither or not ... either.

• Positive idea: moi aussi = me too
• Negative idea: moi non plus = me neither

So:

• Je ne porte pas de T-shirt rose au travail, et Marie non plus. = I don’t wear a pink T-shirt at work, and Marie doesn’t either.

If the first clause were positive, you would use aussi, not non plus.

Yes, that is also possible, but it is a fuller version.

For example:

• Je ne porte pas de T-shirt rose au travail, et Marie n’en porte pas non plus.

The original sentence uses a shorter structure:

• et Marie non plus

That shorter form is very natural and common when the verb idea is understood from the first clause.

French often avoids repeating the noun by using a subject pronoun.

So after mentioning Marie, the sentence continues with:

• elle préfère... = she prefers...

This works just like English:

• Marie doesn’t either; she prefers...

Because French usually needs an article before a plural count noun when speaking in a general but indefinite way.

• des couleurs = some colors / colors
• des vêtements = clothes / some clothes

English often leaves the article out, but French usually does not.

It is a comparative structure:

• discrètes = discreet / subdued
• plus discrètes = more discreet / more subdued

So des couleurs plus discrètes means more subdued colors or more understated colors.

Because it agrees with couleurs, which is feminine plural.

• singular feminine: une couleur discrète
• plural feminine: des couleurs discrètes
• comparative: des couleurs plus discrètes

French adjectives usually agree in gender and number with the noun they describe.

The semicolon links two closely related ideas:

1. I don’t wear a pink T-shirt at work, and neither does Marie.
2. She prefers more discreet colors.

It is a punctuation choice, not a special grammar rule. A period would also be possible:

• Je ne porte pas de T-shirt rose au travail, et Marie non plus. Elle préfère des couleurs plus discrètes.

The semicolon simply shows the ideas are closely connected.

Here it means to wear.

French porter can mean several things depending on context, including to carry or to wear. With clothing, it usually means to wear:

• porter un manteau = to wear a coat
• porter des lunettes = to wear glasses

So in this sentence, it is definitely about clothing.

It can suggest a general habit or usual practice, especially because of au travail.

• Je ne porte pas de T-shirt rose au travail usually sounds like I don’t wear pink T-shirts at work as a general rule or habit.

French present tense often covers both simple present and present continuous depending on context. Here, the workplace context makes the habitual reading very natural.