^this
Due to the scaling of XP with levels it's hard to outlevel class quests just by doing extra quests like PvP, Space and flashpoints, but it is possible. Some bonus quests aren't even available at the end of the planet (Nar Shadaa) and you have to come back to the planet after gaining some levels to get them. I guess to answer your question, yes, if you do all bonus series and other extras your class quests will be green instead of yellow.