Before September 2008 the default link colors on the site were undefined, i.e. depended on the user's browser settings so you could choose whatever scheme you liked (by default, royal blue + dark purple: http://img689.imageshack.us/img689/2293/12804after.gif).

Then they were set to specific values (dark blue + purple: http://img34.imageshack.us/img34/7514/12804before.gif), which was bad for the Message Boards because different users use different Themes (http://imdb.com/help/boards/themes), so they should be able to change the link colors as well.

In September 2010 it was changed again to dull blue + grey (http://img340.imageshack.us/img340/1289/95221236.gif). Many users despised that scheme, so in a few weeks it was changed again to dull blue + dull purple, which is the current scheme (as of late September).