How do I slowly fade from color to black and white?

Use a black and white filter. You then set keyframes so that the filter takes no effect at the beginning of the clip and is at 100% by the end or whenever you want it to get there. If you don't know how to do that, look up keyframing in the manual. They do a great job of explaining the process.

Or, razor blade your edit into two parts, use the Desaturate filter (under Image Control) for the part you want B&W and drop a Cross Dissolve transition between the parts.

Or, drop the 3 way color corrector on the clip and keyframe the desaturation.

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