Agree with Blinkys comments above.
Contrast paints do their magic thing of creating shade, base and highlight when applied on a smooth primer. The GW ones do that very well, although as can be seen – people are finding ways of making their own equivalents.
in my experience, I’ve found that contrast gives me the bright pop-y finish on the contrast primers. Like this Skaven:
and these dwarves:
however, when applied over basic rattle can primers to create a zenith, I can get a very different look. In this case, the contrast tends to “soak” a lot more so it doesn’t behave in quite the same way. However, a black/grey/white zenith mix with contrast over the top gives some great effects, like this vampire:
and this orc:
