Extending a B/W to Color Transition over 2 or more clips PP v.15

Could someone advise me if it's possible to transition from B/W to 100% color using Calculations and keyframes, across more than 1 consequtive clip - and how, please?

If I select more than 1 clip (2 adjoining clips) the Effects window goes blank, and I can only see it again when I deselect all but one clip.

Thanks for any advice.