Sulfuric acid - going brown: Would you please tell me why concentrated sulfuric acid goes brown?

Sulfuric acid - going brown: Would you please tell me why concentrated sulfuric acid goes brown?
Lightning in a test tube: I have a teacher who is planning on performing the 'lightning in a test tube' demonstration in the school lecture theatre.
Cleaning glassware: Dichromic acid glass cleaner waste - I use concentrated sulfuric acid and sodium dichromate in the glass cleaner I make.
Sulfuric Acid in Corrosives Cabinet: A lid was left off 6M sulphuric acid in the corrosive cabinet for some time last year. It reacted with the air and the cabinet started to rust.
This Standard Operating Procedure (SOP) gives a detailed method for how to dilute concentrated sulfuric acid.