A la botiga, jo prefereixo el gris, però la meva amiga vol el blanc.

Yes. A la botiga can be understood as in the shop or at the shop, depending on context.

Catalan often uses a with places where English might use either in or at. So A la botiga is a very natural way to set the scene.

Here it works like a location/topic frame: A la botiga, jo prefereixo el gris... = In/At the shop, I prefer the gray one...

Because Catalan only contracts a + el into al.

So:

• a + el = al
• a + la = a la

Since botiga is feminine, it takes la, so the correct form is a la botiga.

Compare:

• al mercat = to/in the market
• a la botiga = to/in the shop

Yes, it is grammatically optional.

Catalan verbs already show the subject, so prefereixo by itself already means I prefer. That means you could say:

A la botiga, prefereixo el gris...

and it would still be correct.

The pronoun jo is often added for:

• emphasis
• contrast
• clarity

In this sentence, it helps create a contrast with la meva amiga:

• jo prefereixo...
• la meva amiga vol...

So jo feels a bit like I prefer gray, but my friend wants white.

Because preferir is an irregular verb in Catalan.

The first-person singular present form is:

• jo prefereixo = I prefer

This -eixo pattern is common in some Catalan verbs of this type.

A few present-tense forms are:

• jo prefereixo
• tu prefereixes
• ell/ella prefereix
• nosaltres preferim
• vosaltres preferiu
• ells/elles prefereixen

So prefereixo is the correct form, even though it may look unusual at first to an English speaker.

Because here the colors are being used like nouns: the gray one and the white one.

Catalan usually uses the definite article in this kind of situation:

• el gris = the gray one / the gray option
• el blanc = the white one / the white option

There is an implied noun that is not repeated, such as:

• el jersei gris
• el cotxe blanc
• el vestit blanc

Instead of repeating the noun, Catalan just says the color with the article.

They behave like nouns in this sentence, even though they are originally color words that often function as adjectives.

So in:

• prefereixo el gris
• vol el blanc

the meaning is basically:

• I prefer the gray one
• she wants the white one

You can think of them as adjectives used substantively or as color terms used as nouns. Either way, they are standing in for a fuller noun phrase.

Because Catalan normally uses the definite article with possessives.

So the standard pattern is:

• el meu amic
• la meva amiga
• els meus llibres
• les meves sabates

This is different from English, where we simply say my friend without the.

So:

• la meva amiga = my friend

Omitting the article is not the normal everyday pattern here.

Però means but.

The accent is part of the correct Catalan spelling. It marks the stressed vowel and helps distinguish Catalan spelling from similar-looking words in other languages.

So:

• però = but

For an English speaker, the main thing to remember is simply that the Catalan word for but is written però, with the accent.

Vol comes from the verb voler, which means to want.

In this sentence:

• la meva amiga vol el blanc = my friend wants the white one

The present-tense forms include:

• jo vull
• tu vols
• ell/ella vol
• nosaltres volem
• vosaltres voleu
• ells/elles volen

So vol is the correct third-person singular form: she wants.

The word order can be different, but putting A la botiga first is a very natural way to set the scene.

Starting with it gives the sentence a frame like: As for in the shop... or When we’re in the shop...

That is why the comma also makes sense:

• A la botiga, jo prefereixo el gris...

If you moved things around, the sentence would still be understandable, but the emphasis would change. The current order highlights the setting first, then contrasts jo with la meva amiga.