Il cielo diventa arancione al tramonto.

In Italian, the definite article is used more often than in English. So il cielo literally means the sky, and that is the natural way to say it here.

Italian often uses the article with general things or things that are understood from context:

• Il sole = the sun
• La luna = the moon
• Il cielo = the sky

Leaving out il here would sound incomplete or unnatural.

Diventa means becomes or turns.

It comes from the verb diventare = to become.

So:

• Il cielo diventa arancione = The sky becomes/turns orange

This verb is used when something changes state, color, condition, identity, etc.

For example:

• Diventa buio. = It becomes dark.
• L’acqua diventa fredda. = The water becomes cold.

Because diventa emphasizes a change.

• Il cielo è arancione al tramonto = The sky is orange at sunset.
• Il cielo diventa arancione al tramonto = The sky becomes/turns orange at sunset.

Both are possible, but they are not exactly the same:

• è describes a state
• diventa describes the process of changing into that state

In this sentence, diventa highlights the transition in color as the sun goes down.

Diventa is the:

• third person singular
• present indicative
• of diventare

So it matches il cielo, which is singular:

• io divento = I become
• tu diventi = you become
• lui/lei diventa = he/she becomes
• il cielo diventa = the sky becomes

Yes, adjectives do agree in Italian, but arancione is one of those adjectives whose singular form is the same for masculine and feminine.

So:

• il cielo arancione = the orange sky
• la luce arancione = the orange light

In the plural, it changes:

• i cieli arancioni = the orange skies
• le luci arancioni = the orange lights

So arancione is perfectly correct with cielo.

In this sentence, arancione is an adjective describing the color of the sky.

But color words in Italian can also sometimes function as nouns depending on context. Here, though, it is clearly adjectival because it describes cielo after the verb diventare.

A useful thing to remember is that after verbs like essere, diventare, sembrare, Italian often uses an adjective:

• è rosso = it is red
• diventa blu = it becomes blue
• sembra grigio = it seems gray

Al tramonto means at sunset.

The noun tramonto means sunset.

So:

• tramonto = sunset
• al tramonto = at sunset

It refers to the time of day when the sun is going down.

Because al is the contraction of a + il.

Italian commonly combines certain prepositions with definite articles:

• a + il = al
• a + lo = allo
• a + la = alla
• a + i = ai
• a + gli = agli
• a + le = alle

So:

• al tramonto = a il tramonto

But in normal Italian, you use the contracted form al.

Because Italian often uses articles in places where English does not.

Here, al tramonto literally looks like at the sunset, but in natural English we translate it simply as at sunset.

This is very common. Italian and English do not always use articles in the same way, so it is best to learn these expressions as whole chunks:

• al tramonto = at sunset
• all’alba = at dawn
• di notte = at night

Yes. Si fa arancione can also mean turns orange or becomes orange.

Compare:

• Il cielo diventa arancione al tramonto
• Il cielo si fa arancione al tramonto

Both are understandable and natural.
However, diventare is a very straightforward, standard verb for become. For learners, it is often the clearest choice.

That is the most natural word order here:

• Il cielo = subject
• diventa = verb
• arancione = complement
• al tramonto = time expression

Italian word order is flexible, so you could also say:

• Al tramonto, il cielo diventa arancione.

This puts more emphasis on at sunset.
But the original order is neutral and very common.

Yes, cielo is masculine singular, which is why it takes il:

• il cielo = the sky

Its plural is:

• i cieli = the skies

This is useful for agreement and articles.

A rough pronunciation guide:

• cielo ≈ CHYEH-lo
• diventa ≈ dee-VEHN-ta
• arancione ≈ a-ran-CHO-neh
• tramonto ≈ tra-MON-to

A few key points:

• In cielo, ci before e sounds like ch in chess
• In arancione, ci before o also gives that ch sound
• Italian vowels are usually pronounced clearly and fully
• Stress matters: diVENta, aranCIOne, traMONto

In everyday English, yes, that can help you understand it.

• diventa arancione = becomes orange / turns orange / gets orange

But gets orange is less natural in standard English than turns orange or becomes orange. So for learning, the best equivalents are:

• becomes orange
• turns orange

Normally, it describes a general fact or something that happens regularly:

• Il cielo diventa arancione al tramonto. = The sky turns orange at sunset.

The present tense in Italian is often used for general truths, habits, and repeated events, just like in English:

• L’acqua bolle a 100 gradi. = Water boils at 100 degrees.
• Il cielo diventa arancione al tramonto. = The sky turns orange at sunset.

It could also describe what is happening now in the right context, but its most natural reading is general.