Link which explains how to format a letter to Nature I need to write a letter to Nature and a friend of mine said that Nature wants the Author to follow a very precise format. 

He said that the explanation of the structure of the format is an example of a letter in which: there were different colours for the different sections, it also explained what to write in each section and which words use/don't use etc

Does anybody know the link where I can find such information?
Such information is invariably posted on a journal's website, and Nature is no exception. Go to Nature's current issue website, click "For Authors" and "Manuscript Formatting Guide". Under "Letters" there is an "annotated example" which looks like what you are describing.