Hi Onks
I forgot to mention that you can also use the Saturation effect. In this case you can achieve the gradual change from colour to B&W (or B&W to colour) without requiring an overlay clip. (As you can see VP is pretty versatile 🙂) Using this effect is probably simpler.
Use the Saturation effect in a similar way as I described the procedure for changing the clip to Black and White. Set the start keyframe to 0% saturation and further on, the second keyframe set with a saturation level to what you think gives a reasonable appearance.(say, about 25) You can drag the line down to alter the speed of the effect.....Note that the Saturation value can be set to above the the raw level in which case the colours of the clip may be too intense - you will have to check as you do it.The timeline thumbnails will give you some idea.
"... however, I only wish to turn some of the picture back into colour. ..."
I am not exactly clear on what you mean by this when you say "some of the picture".🙂 Do you mean a section of the clip?
If you mean a section of the clip then use the the B&W effect as originally described or the Saturation effect used in the same way.
If you mean a clip turned into B&W but with slight colour (i.e. a weak colour) then just apply the saturation effect at the start of the clip with a setting of around -70. The whole clip will then look pale.
If not either of these please come back with more detail.
Regards
Nat