RAvDEC plot reorders the data before drawing it such that data with lower SUCCESS_LRG1 is draw last (ie on top).  The color at the  moment is rgb(%LRG1, %LRG2, %QSO). So black points have %LRG1=%LRG2=%QSO=0 and white points have %LRG1=%LRG2=%QSO=255.