yeah the longer you're offline, the longer your bar stays blue and a blue bar means double thye EXP.
It also helps to log off in Inns and Taverns especially the one set as your home.
I was once offline for 6 months and my bar was blue for 8 months lol.
I've been doing some serious grinding lately and am now almost lvl 68. I'm going to wait til I'm lvl 70 before I explore Northrend ^^