Because Portuguese nouns have grammatical gender, and coleira is simply classified as feminine.
A native English speaker often expects gender to match real-world sex, but in Portuguese that is usually not the point. Objects also have grammatical gender.
Many nouns ending in -a are feminine, and coleira follows that pattern. But remember: endings are helpful clues, not absolute rules.
Because adjectives in Portuguese usually agree in gender and number with the noun they describe.
Since coleira is feminine singular, the adjective must also be feminine singular:
Compare:
In Portuguese, adjectives often come after the noun, much more often than in English.
So:
That is normal Portuguese word order.
In your sentence, though, the adjective is not directly after the noun because the sentence uses ser:
This structure is:
So it works like The collar is new in English.
Yes, but the meaning changes slightly.
This is the common contrast between ser and estar:
So if you are saying it is literally a new collar, é nova is the most direct choice.
Normally, no.
In standard Portuguese, a singular countable noun used as the subject usually needs an article or another determiner:
Coleira é nova sounds incomplete or unnatural in ordinary speech, except in very special styles such as headings, labels, or very reduced note-like language.
A common Brazilian pronunciation is approximately:
co-LAY-ra
More carefully:
A rough full-sentence pronunciation is:
a co-LAY-ra é NO-va
IPA, if helpful:
Very often, yes: coleira commonly refers to an animal collar, especially a dog collar.
Depending on context, it can also refer more generally to a collar-like strap or band worn around the neck.
So in many everyday situations, a learner will most naturally understand a coleira as the collar for a pet.