El color vermell combina bé amb el blanc, però jo prefereixo el gris.

In this sentence, the colors are being used as nouns, not just adjectives.

• el color vermell = the color red
• el blanc = white / the color white
• el gris = gray / the color gray

Catalan often uses the definite article with color names when talking about them as things or categories. This is more natural than leaving the article out.

So:

• vermell can be an adjective: una samarreta vermella = a red T-shirt
• but it can also act like a noun: el vermell = red

In Catalan, adjectives usually come after the noun.

So:

• color vermell = red color
• cotxe blanc = white car
• paret grisa = gray wall

This is the normal word order. English often puts adjectives before nouns, but Catalan usually puts them after.

Combinar bé amb means to go well with, to match well with, or to combine well with.

So:

• El color vermell combina bé amb el blanc
  = Red goes well with white

Breakdown:

• combina = goes/matches/combines
• bé = well
• amb = with

You will often hear this structure when talking about clothes, decoration, colors, or style.

Examples:

• Aquest vestit combina bé amb aquestes sabates. = This dress goes well with these shoes.
• El blau combina bé amb el negre. = Blue goes well with black.

Because bé and bo are different kinds of words.

• bé = well → adverb
• bo / bona = good → adjective

Here, combina is a verb, so it needs an adverb describing how it combines:

• combina bé = combines well

You would use bo/bona with nouns:

• un bon llibre = a good book
• una bona idea = a good idea

So combina bo would be wrong.

Usually, no, it is not strictly necessary.

Catalan often drops subject pronouns because the verb ending already shows the subject:

• prefereixo already means I prefer

So you could say:

• però prefereixo el gris

That would still be correct.

The pronoun jo is included here for emphasis or contrast:

• ...però jo prefereixo el gris
• ...but I prefer gray

It helps stress the contrast with the previous idea.

Prefereixo is the 1st person singular present tense of preferir:

• jo prefereixo = I prefer

This verb has a stem change in the present tense. Some common forms are:

• jo prefereixo = I prefer
• tu prefereixes = you prefer
• ell/ella prefereix = he/she prefers
• nosaltres preferim = we prefer
• vosaltres preferiu = you all prefer
• ells/elles prefereixen = they prefer

So the -eixo ending is normal here.

Però means but, and it is written with a grave accent: ò.

This accent is part of the correct spelling and also helps show the vowel sound/stress.

Do not confuse it with similar-looking forms in other languages:

• Catalan: però = but
• Spanish: pero = but

So if you are coming from Spanish, remember that Catalan adds the accent here.

Because amb is the normal Catalan word for with.

So:

• amb el blanc = with white

This is just standard Catalan vocabulary. English speakers sometimes learn Catalan through Spanish, so this word may stand out.

Examples:

• cafè amb llet = coffee with milk
• vaig amb ella = I’m going with her
• combina amb això = it goes with that

Because in el color vermell, the noun color is masculine singular, so the adjective is also masculine singular:

• vermell = masculine singular
• vermella = feminine singular
• vermells = masculine plural
• vermelles = feminine plural

Examples:

• el color vermell = the red color
• la camisa vermella = the red shirt
• els cotxes vermells = the red cars
• les flors vermelles = the red flowers

So vermell agrees with color.

Catalan often prefers the article when referring to colors in a general or abstract way.

So these sound natural:

• el blanc
• el gris
• el negre
• el blau

This is similar to saying the white, the gray, though in English we usually just say white and gray.

In Catalan, the article helps show that the color word is functioning as a noun.

Yes. That is very common.

As adjectives:

• una paret blanca = a white wall
• un jersei gris = a gray sweater
• un cotxe vermell = a red car

As nouns:

• el blanc = white
• el gris = gray
• el vermell = red

In this sentence, Catalan uses both patterns:

• el color vermell → color word used with color
• el blanc / el gris → color words used directly as nouns

Yes.

The subject is El color vermell, which is singular, so the verb is singular too:

• El color vermell combina...

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

• Els colors vermell i blanc combinen bé.
  = The colors red and white go well together.

So combina agrees with the singular subject.

A very natural English translation would be:

• Red goes well with white, but I prefer gray.

Catalan often says things a bit more explicitly:

• El color vermell = literally the color red

English usually does not need the color unless we want special emphasis. So while the Catalan sentence is perfectly natural, the most idiomatic English version is often shorter.