transition of a clip from B&W to color

I am new person to FCPx and have what is certainement a simple question to which i don't see the answer.
On a clip i have made it B&W; how, halfway through this clip, can I gradually change it back to colour please ?

Probably you can do that, but it depends on how you made it B&W. Did you use the color board, or did you use an effect?

Thank you for the swift résponse; it was click on the clip and choose B&W effect.

The effect controls can be keyframed in the inspector. At the end of the slider is a button that allows you to add a keyframe. Add one when it's B&W then go to a different point in time in the timeline and change the amount value. Another keyframe will automatically be added.

If you can't key frame e effect then here are two methods you can use.

1. Go to the point where you want to start transitioning to colour. Get out the blade tool and slice the clip there. Remove the effect on the right side and then add a dissolve to the edit.

2. Duplicate the clip by holding down the option key and dragging the clip up over itself. Now remove the effect from the top layer. Now open the inspector and set the opacity of the upper clip to zero. Go to the point where you to transition to colour and add a key frame to the top layer, now go to the point where you the clip to be in full colour and add another keyframe and set the opacity to 100%.

Hope that helps.

Just like that !
Many thanks for taking the time to answer me; much appreciated.