That is odd - but in a way doesn't surprise me.
The darker cernit colours seem to me to be translucent clay just heavily laden with dark coloured mica powders. Even though the package actually claims they are opaque colours - it just seems to me the actual colours are coming from the mica itself rather than added pigments.
So I suppose the Premo pearl is just drowning out the colours in those.
With the brighter blue and green cernit shiny colours, you can see really they are a more traditional pigmented clay with the addition of silvery mica for the sheen. So they just seem to lighten up quite normally with the addition of more pearl.
I guess the next test is to mix the darker colours with more translucent
What a fascinating experiment though - thank you so much for sharing your results with us. I do love a good test-tile experiment with polymer clay!
Emma