First attempt at Prismacolour colour pencils

This pic was inspired by a beautiful picture browse.deviantart.com/art/Main… of a very beautiful kitty. Prismacolor coloured pencils, Dollarama cardstock paper.
Question for any accomplished artists that work in coloured pencils: how do you draw contrasting lighter colours over darker ones while preserving the contrast? I got a nasty little surprise when I found out that the white whiskers could not be drawn over the darker background, even after a coat of fixative. Had to resort to using white opaque ink :-/

Thank you all for the wonderful comments, I am just basking in the warmth of your appreciation (purrr )