الكرسي اصفر.

A common Levantine pronunciation is il-kursi asfar or el-kursi asfar.

A rough English-style guide:

• il/el = the
• KUR-si = chair
• AS-far = yellow

Notes:

• The r is usually a light tap.
• In fast speech, il/el may shrink, so you might hear something closer to l-kursi.
• The standard spelling of the color is أصفر, but in casual writing people often write اصفر.

Because in Arabic, the present tense usually does not use a separate word for is / am / are in sentences like this.

So:

الكرسي اصفر
literally looks like
the chair yellow

but it means:

The chair is yellow.

This is completely normal in both Levantine and Standard Arabic.

Because these are two different structures:

• الكرسي اصفر = The chair is yellow.
  This is a full sentence.

• الكرسي الأصفر = the yellow chair
  This is a noun phrase, not a complete sentence by itself.

So in your sentence, اصفر / أصفر is the predicate of the sentence, not an adjective directly attached to the noun with ال.

Yes. In Arabic, adjectives normally come after the noun.

For example:

• كرسي أصفر = a yellow chair
• بيت كبير = a big house

That is the opposite of English, where adjectives usually come before the noun.

So the word order here is very normal.

Because كرسي is grammatically masculine, so the color must also be in the masculine form.

• masculine: أصفر / اصفر
• feminine: صفرا in Levantine speech

So:

• الكرسي اصفر = The chair is yellow
• الطاولة صفرا = The table is yellow

Even if an object is not biologically male or female, Arabic nouns still have grammatical gender.

In formal standard spelling, أصفر is the correct form.

In casual writing, texting, and informal chat, many Arabic speakers leave out hamza and write اصفر instead.

So:

• أصفر = formal/standard spelling
• اصفر = very common informal spelling

A learner should recognize both.

It could be understood as both.

Simple sentences like this often look the same in Levantine and Standard Arabic when written. The biggest differences are usually:

• pronunciation
• everyday word choice
• some grammar in longer sentences

So the written form الكرسي اصفر is not especially marked as one or the other. In speech, though, a Levantine speaker would pronounce it in a Levantine way.

The usual Levantine way is:

الكرسي مش أصفر
or informally
الكرسي مش اصفر

That means:

The chair is not yellow.

Here, مش is the common negation word for this kind of present-tense sentence.

You can, but it usually changes the meaning.

• الكرسي أصفر = The chair is yellow
• كرسي أصفر = a yellow chair

So without ال, it often sounds like a noun phrase rather than a complete sentence.

If your goal is specifically The chair is yellow, keeping الكرسي is the clearest choice.

The difference is mainly definiteness and sentence vs. phrase:

• كرسي أصفر = a yellow chair
  an indefinite noun phrase

• الكرسي اصفر = The chair is yellow
  a complete sentence with a definite subject

This is a very important distinction in Arabic, because a small change like adding ال can change the whole structure.