Θέλω το γκρι, όχι το μπλε.

Because Greek often leaves out subject pronouns when the verb already shows who the subject is.

Θέλω means I want, so Greek does not need εγώ here. You could say Εγώ θέλω το γκρι, όχι το μπλε if you want extra emphasis on I, but in normal speech Θέλω by itself is enough.

A rough beginner-friendly pronunciation is:

THE-lo to gree, O-hi to bleh

A few useful sound notes:

• Θ sounds like th in think
• γκ at the start of γκρι sounds like a hard g
• χ in όχι is not English h exactly; it is a Greek fricative sound
• μπ at the start of μπλε sounds like b
• The accent mark shows the stressed syllable: Θέ-λω, ό-χι

Here το works like the, but it also helps turn the color word into the grey one and the blue one.

Greek often does this when the noun is understood from context. So:

• το γκρι = the grey one / the grey
• το μπλε = the blue one / the blue

English often needs one, but Greek can use the article plus the color word.

Because the phrase is neuter singular here.

The article must agree with the understood noun, or with a neuter idea such as the one or the color. That is why you see το.

Also, this phrase is the object of θέλω, so it is in the accusative case. For neuter singular, the nominative and accusative article are both το, so the form stays the same.

They are basically color adjectives being used like nouns.

In other words, Greek is using the color word by itself because the actual noun is understood. So these words still behave like color descriptions, but in this sentence they stand for something like the grey one and the blue one.

Usually, no. These are very common indeclinable color words in Modern Greek, which means they often keep the same form.

For example:

• το γκρι φόρεμα
• τη γκρι μπλούζα
• τον γκρι καναπέ
• το μπλε αυτοκίνητο
• τη μπλε τσάντα

The article and noun show the gender and case, while γκρι and μπλε themselves usually stay unchanged.

They are in the accusative, because they are the direct objects of θέλω.

So the structure is:

• Θέλω = I want
• το γκρι = the thing wanted
• όχι το μπλε = contrasting it with another option

As mentioned above, neuter singular το looks the same in nominative and accusative, so you do not see a visible change.

Because όχι is being used for contrast: not the blue.

So:

• Θέλω το γκρι, όχι το μπλε = I want the grey one, not the blue one
• Δεν θέλω το μπλε = I do not want the blue one

Δεν negates a verb.
Όχι often follows a pause and corrects or contrasts something.

Yes, but the meaning changes.

• Θέλω το γκρι = I want the grey one / a specific grey option
• Θέλω γκρι = more like I want grey or I want something in grey

So the article makes the phrase sound more specific.

Yes. Greek word order is fairly flexible, and changing it can shift the emphasis.

For example:

• Θέλω το γκρι, όχι το μπλε = neutral, straightforward
• Το γκρι θέλω, όχι το μπλε = stronger emphasis on the grey
• Όχι το μπλε, το γκρι θέλω = very contrastive, more dramatic

The original sentence is a very natural, standard way to say it.

Then the article would change to match that noun.

For example, with a feminine noun like μπλούζα:

• Θέλω τη γκρι μπλούζα, όχι τη μπλε
• Or with the noun omitted: Θέλω τη γκρι, όχι τη μπλε

With a masculine noun like καναπές:

• Θέλω τον γκρι καναπέ, όχι τον μπλε
• Or with the noun omitted: Θέλω τον γκρι, όχι τον μπλε

So the article changes, but γκρι and μπλε usually stay the same.

In Modern Greek, words of more than one syllable normally show the stressed syllable with an accent mark.

So:

• Θέλω has two syllables, so it gets an accent
• όχι has two syllables, so it gets an accent

But γκρι and μπλε are one-syllable words, and monosyllables usually do not take a written accent mark.