This used to happen too in FCP 7 with some effects. In FCPX’s hierarchy, Colour Board is after whatever filter you apply, unlike 3 way colour correction which was a filter itself.
I remember in the early days watching a program where I used Glowing transitions only to discover that whatever transmition limiter had turned the nice white glow into deep black… not pretty.
Also, white in the Mac’s colour palette is 255 which translates into above legal white level. If you use the “crayons” palette you have to select Mercury white for a legal white, Snow is too bright.
Patrice