Paint brush on top or under

When using the painbtbrush tool, sometimes new lined draw above previous paintbrush lines and sometimes they draw below previous lines.

So like I do some sketching in one color, and then I go to do some cleanup afterwards in another color, and the new lines appear beneath the sketch lines and other times they appear above them, and I cannot for the life of me figure out what parameters decide this.  All of this is taking place on the same layer.  

I am not interested in doing the work on different layers, because onion skinning and all that becomes complicated and annoying.

Thanks for any help, in advance!