If you want your photograph to be black-and-white, with a small area in color, do the following.
1) Go to the Develop module.
2) Select the Adjustment Brush.
3) Double click Effect to zero the settings.
4) Change Saturation to -100.
5) In the Brush section:
• Click A.
• Change Size to a huge brush.
• Change Feathering to 0.
• Deselect Auto Mask.
6) Below your photograph, deselect Show Selected Mask Overlay.
7) Brush everywhere on your photograph.
8) Simply changing the Saturation to -100 usually creates a lackluster black-and-white photograph.8) Adjust Exposure, Brightness, and Contrast, as needed.
9) In the Navigator panel, magnify the photograph so the area-that-you-want-to-be-in-color fills the screen.
To move the photograph around, press and hold on the photograph, and drag.
Or, click and hold on the white rectangle in the Navigator panel, and drag.
10) In the Brush section:
• Click Erase.
• Change Size to a value appropriate to the part of the photograph that you want to be in color.
• Change Feathering to 75 or thereabouts.
• Select Auto Mask if there's a color or tonal difference between—the area-that-you-want-to-be-in-color—and the area-that-you-want-to-remain-black-and-white.
Auto Mask will help you to confine your brushing only on the area-that-you-want-to-be-in-color.
11) Below your photograph, select Show Selected Mask Overlay.
You'll see the orange mask color where you brushed.
12) Brush where you want the color to be.
13) If you used Auto Mask, check for flaws.
Flaws appear as black-and-white specs.
Or, if you're using the orange mask overlay, the flaws are orange.
To correct, use the Erase brush.
If the flaw is in the black-and-white area, use the A brush to correct.