Below is the original image after basic enhancements, which I want turn it B&W, but leave the old lady's walking aid red. In this image you could safely pick the color directly. Risk in that method is that if there is same tone in other parts of the image, those would be also left colored. The method I describe is slightly more complicated, but it will guarantee, that there are no unwanted colored spots left.
Select the B&W tool and draw a mask around the object you want leave colored. You don't need to follow the object's outline, it's enough that there is not same tone within the area. In the B&W tool settings tick the "Invert mask".
The next step is turn also the area around the walking aid B&W. Select a second B&W tool and in the color settings select the eye-dropper. Click on the walking aid and tick "Invert" – and there you are!
I like this image more, when it's pure B&W, but I selected this to show the power of LZ on an object that has a very complex shape. It would a huge task to draw a mask that follows the outline of the walking aid.
If you use the same method with other tools than B&W they may not sum as nicely as B&W. My suggestion would be to remove the transition zone in the mask and link the mask to the tool's second appearance. Then you would be applying the same transition only once to each pixel.
