adding color to black and white photos

Photos does not support this natively. Sometimes you can bring out selective colors using the color picker tool, but it will only work, if the object in question is the only one that has this color.

I am occasionally using the Color Strokes.app. It is right now bargain priced for just 99 cents. (ColorStrokes on the Mac App Store)

You can brush color in and remove color selectively. You can call this application from Photos using the menu "Image > Edit With > Others".

Here is an example for touching up a photo with ColorStrokes:

Here is an example of the best I could do with only the editing tools in Photos:

• First I took the saturation out of the photo using the color adjustment. A little color needs to stay however, or Photos cannot find the yellow buoy.
• Then I sampled the color of the buoy withe the "Selective Color" picker and increased the saturation of this color. The Range slider needs to be set to a high value.

If you export the adjusted photo and then reimport it to Photos, you can increase the saturation of the colored item by using Selecting Cloud again.

If you removed the color from the photo with Photos' editing capability just do a revert to original to bring it back. However, to get a combination of color and B/W in the same photo you'll need to use a 3rd party image editor that can do that like Photoshop Elements, GraphicConverter or GIMP:

You need an editor that can make a specific selection and then desaturate it or the inverse of it.

If you're starting out with a B/W image that would take a lot more skill to add color than I've accustomed to.