لون الباب دهبي.

A common Egyptian pronunciation is lōn el-bāb dahabi.

A rough breakdown:

• لون = lōn
• الباب = el-bāb or il-bāb
• دهبي = dahabi

So the whole sentence sounds roughly like: lone el-baab dahabi.

It literally means color of the door or the door’s color.

This is a very common Arabic structure called an idafa:

• لون = color
• الباب = the door

In Arabic, the possessed thing comes first, so:

• لون الباب = the color of the door not the door of the color

Because Arabic usually leaves out is / am / are in the present tense.

So:

• لون الباب دهبي literally looks like the color of the door golden
• but it naturally means The color of the door is golden

This is completely normal in both Egyptian Arabic and Standard Arabic.

Because in an idafa phrase, the first noun usually does not take الـ, even when the whole phrase is definite.

So:

• لون باب = a door’s color / the color of a door
• لون الباب = the color of the door

The definiteness comes from the second word here, الباب, and it makes the whole phrase definite.

Because دهبي is the predicate of the sentence, and predicates in sentences like this are usually indefinite.

So:

• لون الباب دهبي = The color of the door is golden

If you said الدهبي, it would sound more like the golden one, which is not what you want here.

A useful contrast:

• الباب الدهبي = the golden door
  Here الدهبي is an adjective directly describing الباب
• لون الباب دهبي = the door’s color is golden
  Here دهبي is the predicate

It is a nominal sentence.

That means it starts with a noun phrase, not a verb.

Structure:

• لون الباب = the topic / subject
• دهبي = the predicate

So the pattern is basically:

[noun phrase] + [description]

This is one of the most common sentence types in Arabic.

Because it matches لون, which is a masculine singular noun.

Even though door might feel like the important idea in English, grammatically the sentence is about the color, not directly about the door.

So:

• لون = masculine singular
• دهبي = masculine singular

If you directly described a feminine noun, the adjective would usually be feminine:

• الشنطة دهبية = The bag is golden

Yes, basically.

In Standard Arabic, golden is usually written ذهبي.
In Egyptian Arabic, people often say and write دهبي in everyday usage.

So for a learner:

• ذهبي = the more Standard Arabic spelling
• دهبي = a common Egyptian-style form

Both point to the same meaning here: golden / gold-colored.

Because this is Egyptian Arabic, not formal Standard Arabic pronunciation.

In Standard Arabic:

• الباب is pronounced al-bāb

In Egyptian Arabic:

• it is usually pronounced el-bāb or il-bāb

So the spelling stays the same, but the pronunciation changes.

Yes. A very common Egyptian way is:

• الباب لونه دهبي = The door, its color is golden / The door is golden-colored

This is extremely natural in spoken Egyptian.

You might also hear:

• الباب دهبي = The door is golden

That version is simpler, but لون الباب دهبي is more explicit because it specifically talks about the color.

Not necessarily.

Usually دهبي means golden or gold-colored, not literally made of real gold.

So in most everyday situations:

• لون الباب دهبي = The door’s color is golden not
• The door is made of gold

Context decides, but by default people understand it as a color description.