[hamza (OP)]

Why is the colour of rust over metal surfaces brown

[eric l]

• A brown metal surface is probably painted or coated. I don't recall a metal (or alloy) that would show a brown face when sawn through.  Even bronze can hardly be called "brown".  And there are so many shades of brown, "rust" is only one of them.
• "Rust" is a word used for iron or iron alloys.  The oxide on other metals will go by other names.

 

[JimBob]

Rust is iron oxide - it is redish brown. It is most likely what you are looking at.

[hamza (OP)]

But the "rust" (the corrosion and not the colour) on aluminium is also brown.. My question is that why does it appear to berusty in colour?

[eric l]

Aluminiumoxide (by "natural" oxidation) is almost colourless.  Brown alminiumoxide is by an electrophoresis process ("anodization")
What the colour of  substance will be is related to structure, free electron pairs and such things.  and there are differences between

• colour by emmision
• colour by transmission
• colour by reflexion

It is related to the possibility for electrons to move to a higher energy orbital (or to fall back from that).