If you like you're clothes matching or coordinating, people are curious of what colors are the same. Nike likes to re-release colors every few years. They eventually just keep rotating them. So, if they recently re-released the same Chlorine Blue from 2007 (Nadal/Federer), but someone cannot remember for sure, if their top is Chlorine Blue, they can just check the tag and verify the code here for reference.
Exactly what stangaele was asking for here: http://tt.tennis-warehouse.com/showthread.php?t=362039
. He had the tag and could referenced it here.
Also if you're buying an old shirt here or from the bay, you can ask for the color code and reference the time frame it was released.