It's been said that certain social policies benefits are only available during certain eras. Also, as far as I know, they have never titled the tech tree in such a way with eras at the top. Maybe now eras are defined by date, so you can only research certain techs, in certain eras, and once you move on, they are locked out for good. Early techs would be easier, then the farther along the more complicated choices spring up. And you can research into the next eras techs, but you could miss vital ones by going too far ahead in only one path.
For example the picture above. The user has teched up to gunpowder the farthest, then a new era started. The green techs are the available techs to start from now. Maybe if he had Missed a path somehow leading to the new era those techs would be black, say if he went on a straight line to gunpowder, and kept on going. So anyways, green are the new start, as you can see he already went into gunpowder/new era so there isn't green there obviously. Black has yet to be reaserched/unavailable. The reason one title bar is blue, and the others yellow, is to show the player what age they are in. Yellow being completed/current, blue meaning not that era yet, so possibly slow down teching up in here, and try to get important techs in your era fool!