Προτιμώ ένα μονόχρωμο φόρεμα για το γραφείο και όχι κάτι πολύ έντονο.

Προτιμώ is the 1st person singular present tense of the verb προτιμώ, meaning I prefer.

So:

• προτιμώ = I prefer
• προτιμάς = you prefer
• προτιμά = he/she/it prefers

The ending -ώ is a common present-tense ending for some Greek verbs in -ώ.

Because φόρεμα is a neuter singular noun, and in Greek the article and adjective must agree with the noun in gender, number, and case.

So here:

• ένα = neuter singular a
• μονόχρωμο = neuter singular single-colored / plain
• φόρεμα = neuter singular dress

If the noun were feminine, the forms would change. For example:

• μια μονόχρωμη φούστα = a plain skirt

Μονόχρωμο literally means one-colored:

• μόνο- = single / only
• -χρωμο = colored

In natural English, for clothing, it often means:

• plain
• solid-colored
• single-colored

So it suggests a dress without a busy pattern or multiple colors.

Για usually means for, and here για το γραφείο means for the office or for wearing at the office.

It describes the purpose or setting of the dress:

• ένα φόρεμα για το γραφείο = a dress for the office

This is very natural in Greek when talking about clothes that are suitable for a particular situation.

Greek uses the definite article much more often than English does.

So where English might say:

• for the office or even just understand office wear

Greek normally says:

• για το γραφείο

Here το γραφείο means the office, but in context it often really means the workplace / office environment.

Both are in the accusative singular.

Why?

• φόρεμα is the direct object of Προτιμώ
• γραφείο comes after για, which takes the accusative

So grammatically:

• Προτιμώ ένα μονόχρωμο φόρεμα
• για το γραφείο

A useful thing to remember: for many neuter singular nouns, the nominative and accusative forms look the same, so you do not see a visible change here.

Και όχι means and not, but in this kind of sentence it works like rather than or as opposed to.

So the structure is:

• I prefer X, and not Y
• more naturally in English: I prefer X rather than Y

Here it creates a clear contrast:

• ένα μονόχρωμο φόρεμα = the preferred option
• κάτι πολύ έντονο = the rejected option

Because after κάτι meaning something, Greek usually uses a neuter singular adjective.

So:

• κάτι έντονο = something bold / something striking
• κάτι όμορφο = something beautiful
• κάτι περίεργο = something strange

This happens because κάτι refers to an unspecified thing, and Greek treats that as neuter.

In this context, έντονο does not mean just intense in a general sense. For clothes, it usually means something like:

• bold
• loud
• flashy
• striking

So κάτι πολύ έντονο suggests clothing that stands out too much, perhaps because of bright colors, dramatic design, or a strong visual effect.

Here πολύ means very, so it is functioning as an adverb modifying the adjective έντονο.

• πολύ έντονο = very bold / very striking

This is different from πολύς, πολλή, πολύ, which can also be an adjective meaning much / many.
In this sentence, it is the adverb very.

Yes. Greek word order is more flexible than English word order.

The given sentence is a very natural, neutral way to say it:

• Προτιμώ ένα μονόχρωμο φόρεμα για το γραφείο και όχι κάτι πολύ έντονο.

But you could move parts around for emphasis. For example:

• Για το γραφείο προτιμώ ένα μονόχρωμο φόρεμα...

That puts extra focus on for the office.
Even so, the original version is probably the most straightforward and natural for a learner to use.