Το ασπρόμαυρο πουκάμισο είναι ωραίο, αλλά σήμερα τελικά επιλέγω το μονόχρωμο.

Because πουκάμισο is a neuter noun in Greek. Greek nouns have grammatical gender, and you learn each noun together with its article:

• ο = masculine
• η = feminine
• το = neuter

So you should learn the word as το πουκάμισο = shirt.

Unlike English, grammatical gender in Greek is a basic part of the noun, so it affects articles, adjectives, and sometimes pronouns too.

Because they all agree with a neuter singular noun.

In Greek, adjectives must agree with the noun they describe in:

• gender
• number
• case

Since πουκάμισο is neuter singular, the adjectives are also in the neuter singular form:

• ασπρόμαυρος / ασπρόμαυρη / ασπρόμαυρο
• ωραίος / ωραία / ωραίο
• μονόχρωμος / μονόχρωμη / μονόχρωμο

So:

• το ασπρόμαυρο πουκάμισο
• είναι ωραίο
• το μονόχρωμο

all use the neuter singular form.

That is one of the normal ways Greek forms an attributive adjective.

A very common pattern is:

• article + adjective + noun

So:

• το ασπρόμαυρο πουκάμισο

means the black-and-white shirt.

Greek can also place the adjective after the noun, but then the structure usually changes, often with the article repeated:

• το πουκάμισο το ασπρόμαυρο

That version can sound more emphatic or contrastive, depending on context.

So in your sentence, το ασπρόμαυρο πουκάμισο is just the straightforward, natural order.

Because Greek often lets an article + adjective stand on its own when the noun is already understood from context.

So:

• το μονόχρωμο

literally means something like:

• the solid-colored one
• the plain one

Here, the missing noun is clearly πουκάμισο, so the full idea is:

• το μονόχρωμο πουκάμισο

but Greek does not need to repeat it.

This is very common in everyday Greek. English does something similar with the red one, the big one, the expensive one.

Good question. It actually is in the accusative, because it is the direct object of επιλέγω.

But in the neuter singular, the nominative and accusative forms are usually identical. So:

• nominative: το μονόχρωμο
• accusative: το μονόχρωμο

They look the same.

That is why:

• Το ασπρόμαυρο πουκάμισο είναι ωραίο
  uses a nominative subject

but

• επιλέγω το μονόχρωμο
  uses an accusative object

and both still appear as το ... -ο.

Greek usually drops subject pronouns when they are not needed.

The verb ending already tells you who the subject is:

• επιλέγω = I choose
• επιλέγεις = you choose
• επιλέγει = he/she/it chooses

So Greek does not need εγώ here.

If you did say εγώ επιλέγω, that would usually add emphasis, something like:

• I’m the one choosing
• I choose, not someone else

But in a neutral sentence, Greek normally leaves the subject pronoun out.

In Greek, the present tense often covers both:

• simple present: I choose
• present progressive: I am choosing

The exact English translation depends on context.

So σήμερα τελικά επιλέγω το μονόχρωμο can naturally mean:

• Today I choose the solid-colored one
• Today I’m choosing the solid-colored one
• Today I’m going with the solid-colored one

Greek does not always need a separate form for the English-style progressive.

τελικά means something like:

• in the end
• ultimately
• after all
• finally, in the sense of a final decision

In this sentence, it suggests that there was some comparison, hesitation, or thought process, and this is the conclusion:

• the black-and-white shirt is nice,
• but today, in the end, I choose the solid-colored one.

So τελικά adds the idea of a settled outcome, not just time.

They describe different kinds of appearance.

• ασπρόμαυρο = black-and-white
• μονόχρωμο = single-colored, solid-colored, plain

So:

• ασπρόμαυρο πουκάμισο = a shirt with black and white in it
• μονόχρωμο πουκάμισο = a shirt in one color only

Also, ασπρόμαυρο is a compound adjective made from:

• άσπρο = white
• μαύρο = black

Both άσπρος and λευκός can mean white, but they are not used in exactly the same way.

• άσπρος is very common in everyday speech
• λευκός can sound more formal, technical, or stylistically elevated in some contexts

The compound ασπρόμαυρο is the normal everyday word for black-and-white. It is basically a fixed expression.

So even if you know λευκός, you should still learn ασπρόμαυρος as the standard adjective here.

Not necessarily. ωραίος / ωραία / ωραίο is a very common adjective with a broad meaning range, including:

• nice
• pretty
• beautiful
• lovely
• good-looking

For an object like a shirt, είναι ωραίο most often means something like:

• it’s nice
• it looks good

So in this sentence, ωραίο is very natural and everyday, not overly dramatic.

Yes, Greek word order is fairly flexible, especially with adverbs.

In this sentence:

• σήμερα sets the time: today
• τελικά adds the idea of final decision: in the end

So σήμερα τελικά επιλέγω sounds natural as:

• today, in the end, I choose...

Other orders are possible, but the emphasis changes a bit. For example:

• Τελικά σήμερα επιλέγω το μονόχρωμο
• Σήμερα επιλέγω τελικά το μονόχρωμο

These are all understandable, but the original order is smooth and idiomatic. It first gives the time frame, then the sense of conclusion, then the verb.