I updated to Build 452 and opened a project that had several Chroma Key clips.
They weren't keyed out.
I started a new project with the same clips figuring it was something I did to cause it not to work. I had the same problem. I could not key with VP's Chroma Key using the color picker.
I took the same clips into VP16, and there was no problem doing the same thing, and VP's Chroma Key worked.