There's a thread about this in the forum.
Your web browser tracks your web history, and it distinguishes between links you've visited and links that you've never visited.
By default, new links appear blue while visited links appear purple. A website can override these styles with a style sheet, and in the past it would seem that HubPages made both types of links appear the same blue color. Now, for whatever reason, they've changed the color of the visited links to purple.
So there's no significance to the different color, except for the fact that you've visited that link before. If you clear your browsing history or open up HubPages in a different browser that you never use, you should see all of your links in blue (at least, until you click on them and visit them again).