La bufanda roja combina con mis vaqueros.

Because adjectives in Spanish agree in gender and number with the noun they describe.

• Bufanda is a feminine singular noun (la bufanda).
• The color adjective rojo has to match that:
  • Masculine singular: rojo
  • Feminine singular: roja
  • Masculine plural: rojos
  • Feminine plural: rojas

So we say la bufanda roja (the red scarf).
If the noun were masculine, e.g. el jersey (the sweater), it would be el jersey rojo.

In Spanish, the default position for descriptive adjectives (like colors) is after the noun:

• la bufanda roja = the red scarf
• unos pantalones largos = some long trousers

Putting a descriptive adjective like a color before the noun is unusual and can sound poetic, literary, or marked for special emphasis.
So la roja bufanda is grammatically possible, but in everyday speech it sounds odd or very stylized. Native speakers will practically always say la bufanda roja.

The subject is la bufanda roja.

• La bufanda roja (the red scarf) = she/it in English.
• The verb combinar is conjugated in the present indicative:
  • yo combino
  • tú combinas
  • él/ella/usted combina
  • nosotros combinamos
  • vosotros combináis
  • ellos/ellas/ustedes combinan

Since the subject is la bufanda (third person singular), the correct form is combina:

• La bufanda roja combina con mis vaqueros.
  The red scarf goes with my jeans.

If the subject were plural, the verb would be plural:

• Las bufandas rojas combinan con mis vaqueros.
  The red scarves go with my jeans.

In this context, combinar con means “to match” / “to go well with” in the sense of colors or style fitting together:

• Esta camisa combina con tus zapatos.
  This shirt goes with your shoes.

Yes, combinar con is a very common, natural way in Spain to talk about clothes or colors that look good together.

Other expressions with a similar meaning:

• hacer juego con
  
  • La bufanda roja hace juego con mis vaqueros.
• pegar con (more informal/colloquial)
  
  • La bufanda roja pega con mis vaqueros.

But combinar con is standard and works perfectly here.

The verb combinar, when used with the meaning “to match / to go well together”, needs the preposition “con”:

• X combina con Y = X matches/goes (well) with Y.

So the correct structure is:

• La bufanda roja combina con mis vaqueros.

If you say combina mis vaqueros, it changes the meaning to something like “combines my jeans (with something)”, where jeans are a direct object, which is not what you want here.

Yes, un vaquero can mean “a cowboy” in general Spanish, but in Spain, los vaqueros very commonly means “jeans” (denim trousers).

Usage by region:

• Spain:
  • los vaqueros = jeans
  • You’ll also hear los tejanos in some areas (especially Catalonia) for jeans.
• Latin America (varies by country):
  • los jeans
  • los pantalones de mezclilla (Mexico)
  • los pantalones vaqueros (in some regions)

So in Spain, mis vaqueros is a normal way to say my jeans.

In Spanish, like in English, trousers/jeans are normally treated as plural because they have “two legs”:

• los pantalones = (a pair of) trousers
• los vaqueros = jeans
• las gafas = glasses

So we say:

• mis vaqueros = my jeans
• Estos vaqueros son cómodos. = These jeans are comfortable.

A singular like un vaquero is possible but refers to one item of that type in a more abstract sense (e.g. “a pair of jeans” in a store context), and in practice people almost always use the plural.

They all involve vaqueros (jeans), but the nuance and structure differ:

• mis vaqueros

  • Possessive adjective: mi / mis in front of the noun.
  • Direct, neutral way to say “my jeans.”
  • Used most commonly: mis vaqueros.

• los vaqueros

  • Just “the jeans,” no explicit owner.
  • Could be any jeans, or whose jeans is clear from context:
    • Los vaqueros combinan con todo. = Jeans go with everything (general statement).
    • Los vaqueros están en la silla. = The jeans are on the chair (maybe obvious they’re yours).

• los vaqueros míos

  • Possessive pronoun-like form after the noun: mío(s), tuyo(s), suyo(s) etc.
  • Adds emphasis: “the jeans of mine” / “those jeans that are mine.”
    • La bufanda roja combina con los vaqueros míos, no con los tuyos.
      The red scarf goes with my jeans, not yours.

In your sentence, mis vaqueros is the simplest and most natural.

Both are grammatically correct; it’s just a difference in what you emphasize.

• La bufanda roja combina con mis vaqueros.

  • Emphasis on which scarf: “The red scarf goes with my jeans.”
  • It doesn’t say who owns the scarf; it could be yours, someone else’s, or just a specific red scarf we both know.

• Mi bufanda roja combina con mis vaqueros.

  • Emphasis on possession: “My red scarf goes with my jeans.”
  • Makes it clear the scarf belongs to you.

In many real-life situations where you’re talking about your own clothes, mi bufanda roja would be very natural.

In Spanish, singular countable nouns almost always need an article (or another determiner like mi, esta, alguna).

• La bufanda roja = the red scarf
• Mi bufanda roja = my red scarf
• Esta bufanda roja = this red scarf

Saying Bufanda roja combina con mis vaqueros without any article or determiner sounds ungrammatical in normal Spanish.

So you must use some determiner:

• La bufanda roja combina con mis vaqueros.
• Mi bufanda roja combina con mis vaqueros.
• Esta bufanda roja combina con mis vaqueros.

That sounds wrong or very odd in this context.

• Combinar is normally not reflexive when you mean “X matches/goes with Y.”
• The standard structure is:
  X combina con Y.

Using it reflexively (se combina) might appear in other, more abstract uses (e.g. “elements combine with each other”), but for clothes and colors the natural form is non-reflexive:

• La bufanda roja combina con mis vaqueros. ✅
• La bufanda roja se combina con mis vaqueros. ❌ (unnatural for clothes matching)

You’d have to make both adjectives agree with their nouns in gender and number:

• La bufanda roja combina con mis vaqueros azules.
  The red scarf goes with my blue jeans.

Explanation:

• bufanda is feminine singular → roja
• vaqueros is masculine plural → azules (blue in plural form)

More examples:

• Las bufandas rojas combinan con mis vaqueros azules.
  The red scarves go with my blue jeans.
• La bufanda roja combina con mi vaquero azul.
  The red scarf goes with my blue jean (one pair of jeans; less usual, but grammatically fine).