Produ e the portjon that you want to be b&w. I would go past about 10 se onds. Now, put the color clip in one track and the b&w clip in another track. Fade out the b&w clip as you fade in the color clip. __________________________________
CORNBLOSSOM

Another way is to use keyframes for the saturation: 0 (zero) at the start of the clip, 100 at the end of the clip.

I'm going to have to toy around with this one to see what I can come up with. Leave it with me! I have an idea that might work but I have to try it first, "suck it and see!" If you get my meaning.

I've been toying with this since reading the question and I think I have a solution, as follows:
1) Lay the clip you wish to apply the effect to, on the timeline, also lay the clip on the overlay(PiP) track. Naturally the two should be synchronised.
2) Determine the point at which you wish the effect to occur(transition from monochrome to colour) and split BOTH images at that point. Make a second split on the overlay track only, at 5 seconds before the split on both tracks.
3) On the main image, highlight the portion before the split and click on fix/enhance, select colour adjustment and slide the saturation back to monochrome(to the left). Now you have the monochrome portion of your clip.
4) On the overlay track, delete the portion before the first split and after the second, leaving only that 5-second portion which will be used to create the transition. Stretch the image out so it fills the screen.
5) Select Modify, then select animation, from there select fade-in. If you notice a slight jump in the image, this can be corrected by pulling the overlay at end of the effect(Click and drag to stretch the length, effectively "restores" some of what was deleted).
You now have a completed transition, all that's left is to "produce" it!

Why the hazzle with two identical clips on two tracks? As I said before you can use keyframes to set different values of saturation at different points in the clip. Far more easy than working with two different clips IMHO.

It's a matter of personal.preference. Your method will certainly work, but keyframes easier than fading two clips? I disagree. More than one way to do anything. Use the way that works best for you. __________________________________
CORNBLOSSOM

Hi Spekkyboy1,

I have attached 3 photos showing how I achieved going from B&W to Color, it is using keyframes and the Black & White Effect. Place your clip on timeline, place B&W effect on line below it. Select B&W effect so it is blue high lighted and then click keyframes. Notice I placed them at 1 second intervals and set the keyframes (red) and the degree setting also changes, moving back to 100 then to zero (0). Hope this helps.

Dave HP Envy 17
Win10 Pro (64-bit) Intel i7 Core - 4510U @ 2.00GHz 2.60GHz
16GB ram
PowerDirector 11 Ultra 64-bit ver. 11.0.0.3625
Cyberlink Power2Go Deluxe 7.0.0.1827
PowerShot SX40HS
Canon Vixia

Here is a small attached video showing the effect from above post.

Dave HP Envy 17
Win10 Pro (64-bit) Intel i7 Core - 4510U @ 2.00GHz 2.60GHz
16GB ram
PowerDirector 11 Ultra 64-bit ver. 11.0.0.3625
Cyberlink Power2Go Deluxe 7.0.0.1827
PowerShot SX40HS
Canon Vixia