No quiero la roja; prefiero la azul.

Because in Spanish, the article la can stand in for a feminine singular noun that has already been understood from context.

So la roja literally means the red one and la azul means the blue one.

For example, if you are choosing between two shirts, No quiero la roja; prefiero la azul means I don’t want the red one; I prefer the blue one.

Spanish does this very naturally:

• el rojo = the red one (masculine)
• la roja = the red one (feminine)
• los rojos = the red ones
• las rojas = the red ones

Because the adjective has to agree with the noun it refers to in gender and number.

Here, the missing noun is understood to be feminine singular, so the adjective must also be feminine singular:

• rojo = masculine singular
• roja = feminine singular

For example:

• el coche rojo = the red car
• la camisa roja = the red shirt

In your sentence, the noun is omitted, but the agreement stays:

• la roja = the red one

Not all Spanish adjectives change form for gender.

Some adjectives, like rojo, change:

• rojo / roja

But others, like azul, stay the same in both masculine and feminine singular:

• el coche azul
• la camisa azul

So:

• la azul is correct
• la azula is not correct

In the plural, azul does change for number:

• el azul
• la azul
• los azules
• las azules

Because the sentence is not just naming colors; it is referring to specific things.

La roja and la azul mean:

• the red one
• the blue one

The article la is necessary because it shows that you mean a specific item, not the abstract color itself.

Compare:

• Me gusta el rojo. = I like red. (the color red)
• Quiero la roja. = I want the red one.

In standard Spanish, negation usually goes directly before the verb.

So:

• Quiero la azul. = I want the blue one.
• No quiero la roja. = I don’t want the red one.

This is the normal pattern:

• No entiendo. = I don’t understand.
• No tengo tiempo. = I don’t have time.

Because Spanish often leaves out subject pronouns when they are not needed.

The verb endings already show who is doing the action:

• quiero = I want
• prefiero = I prefer

So yo is optional here:

• No quiero la roja; prefiero la azul.
• Yo no quiero la roja; yo prefiero la azul.

The second version is grammatical, but it sounds more emphatic. Normally, Spanish prefers to omit yo unless you want contrast, emphasis, or clarity.

Because preferir is a stem-changing verb.

In the present tense, the e in the stem changes to ie in most forms:

• prefiero
• prefieres
• prefiere
• preferimos
• preferís
• prefieren

So:

• yo prefiero = I prefer

This is a very common pattern in Spanish. Similar verbs include:

• quiero from querer
• entiendo from entender
• pienso from pensar

They express two slightly different ideas:

• quiero = I want
• prefiero = I prefer

So the sentence does two things:

1. It rejects one option: No quiero la roja
2. It chooses another option: prefiero la azul

This sounds very natural in Spanish and in English:

• I don’t want the red one; I prefer the blue one.

The semicolon links two closely related clauses:

• No quiero la roja
• prefiero la azul

It shows a stronger break than a comma, but not as strong as a full stop.

So all of these are possible, depending on style:

• No quiero la roja; prefiero la azul.
• No quiero la roja, prefiero la azul.
• No quiero la roja. Prefiero la azul.

The semicolon is a neat written way to connect two related ideas.

Yes, that is grammatical, but it means something slightly different.

• No quiero la roja; prefiero la azul = I don’t want the red one; I prefer the blue one.
• No quiero la roja, quiero la azul = I don’t want the red one, I want the blue one.

Prefiero emphasizes preference between options.
Quiero simply states what you want.

If you are comparing two choices, prefiero is often the more natural word.

Yes, it sounds completely natural in Spain.

It is a very normal way to choose between two feminine items when the noun is already understood from context. For example:

• dresses
• shirts
• bags
• doors
• options

A speaker in Spain would easily understand it as:

• I don’t want the red one; I prefer the blue one.

Then the article and any adjective that changes for gender would also become masculine.

Your sentence would become:

• No quiero el rojo; prefiero el azul.

Here:

• el rojo = the red one (masculine)
• el azul = the blue one (masculine)

So the pattern is:

• feminine: la roja / la azul
• masculine: el rojo / el azul

You would make both the article and the adjective plural.

For feminine plural:

• No quiero las rojas; prefiero las azules.

For masculine plural:

• No quiero los rojos; prefiero los azules.

Notice:

• roja → rojas
• azul → azules

So Spanish agreement affects:

1. the article
2. the adjective
3. sometimes the noun, if it is expressed