الفستان اصفر.

A common Egyptian-style pronunciation is el-fostān asfar.

A few helpful notes:

• الـ is usually pronounced el- in Egyptian Arabic.
• In الفستان, the l of el- is pronounced normally because ف is not a sun letter.
• ص in اصفر is a heavier, emphatic s sound, so asfar is not exactly the same as an English s.

A more careful, standard-style reading would be closer to al-fustān aṣfar, but for Egyptian speech el-fostān asfar is a good guide.

Because Arabic usually leaves out the verb to be in the present tense.

So الفستان اصفر literally looks like the dress yellow, but it naturally means The dress is yellow.

This is very normal in both Standard Arabic and Egyptian Arabic.
If you want a past meaning, Arabic uses a verb, for example كان for was.

Because الفستان is the subject, the dress, and اصفر is the predicate adjective, yellow.

In Arabic, when an adjective is used like is yellow, it normally does not take الـ.

So:

• الفستان اصفر = The dress is yellow
• الفستان الأصفر = the yellow dress

That small difference is very important.
Without الـ on the adjective, the sentence is describing the dress.
With الـ on the adjective, the adjective becomes part of the noun phrase.

You would say الفستان الأصفر.

Here both words are definite:

• الفستان = the dress
• الأصفر = the yellow

In Arabic, an adjective that directly describes a noun usually comes after the noun and matches it in definiteness. So:

• الفستان اصفر = The dress is yellow
• الفستان الأصفر = the yellow dress

This is one of the most common things English speakers need to get used to.

In Arabic, grammatical gender does not always match what an English speaker might expect.

The noun فستان is grammatically masculine, so the adjective is also masculine: اصفر.

That is why the sentence is not using a feminine form.

If the noun were grammatically feminine, the color adjective would change too. In Egyptian Arabic, a feminine form like صفرا is common. So the agreement depends on the Arabic noun’s gender, not on English ideas about the object.

The standard spelling is أصفر with a hamza.

However, in casual Egyptian writing, people very often leave hamzas out, so اصفر is extremely common and fully understandable.

So a good rule is:

• أصفر = more standard spelling
• اصفر = very common informal spelling

You will see both.

Yes, الفستان اصفر is natural and easy to understand.

In everyday Egyptian Arabic, people may also say الفستان لونه أصفر, literally the dress, its color is yellow. That version can sound a bit more conversational in some contexts.

So both are useful:

• الفستان اصفر = simple and correct
• الفستان لونه أصفر = also very common in speech

As a learner, you should recognize and understand both patterns.