Undestanding game analysis

Please forgive my ignorance but there must be a legend for this somewhere. When I read an article where a game is described move by move with commentary I become confused. The move(s) actually played are usually in "bold black" - understood. Then these are optionally followed by variations or other moves to be considered. Some of these move sequences are in "light grey" and some are in "red". What is the significance of the coloring?

Thanks, and many apologies if there was somewhere obvious I should have gone to get this answer without bothering the forum (a link in that case please).

Graham....

An analysis of a game has variations(alternative lines).Some of the variations has also variations(sub variations).

With light grey are the variations and with red are the  sub-variations(if I am not mistaken) 

Imagine a complicated variation with many sub-variations.It would certainly confuse the reader.The various colors makes it it easier for the reader to navigate and know exactly what he/she is watching.