The "weight" factor...
When discussing Reds/Burgundies, etc....
I sometimes have a customer pick up a Qt. of Magenta colorant.
Then, I have them pick up a Qt. of KX, the TI02 "White" colorant.
Their eyes get big, and I usually hear a little "oooff"!
Then I start telling them about the differences in the tint-bases; and how each step down (deeper base) gets more translucent (meaning LESS of the heavy TI02).
Increasing amount of White have to be left out so the colors can be more intense/brighter.
The tradeoff obviously is sheer colors that need the help of a colored backdrop (primer) that'll cut the background reflection of a white surface thru a sheer color/tint-base.
Grays work well 95% of the time because...
Gray reduces the amount of light reflected. Simple as that!
Faron