Mavericks Tag colors
You can't.

The seven colours are unchangeable, because this is needed to provide backwards compatibility with earlier versions of OS X (where they were called labels, you could only have one, and you couldn't change the colour).

If you want a detailed explanation of why tags were implemented this way, and why that restricts the tag colours to the default, then I suggest reading page 9 of John Siracusa’s Mavericks review.