Mixdown Help...Master Out Volume and Drum Rack Master Out

[A-Plus]

When I'm doing the mix down on a song, I'm assuming that I don't want the master track level red lining. Why does it red line even when all my other tracks are in the green? do I want to lower the master level so that it's in the green, and then let mastering take care of the rest? or do I leave the master level in the red line since all my other tracks and instruments are in the green?

also, when using a drum rack, I get the same thing. all the devices in the drum rack can be in the green, but if the master level of the drum rack is left at 0db, it still red lines. why is this?

If someone could please clarify this for me, it would make my mix down a little easier to complete.

Thank-you in advance