1) Open the photo in gallery
2) Click the 3 dots and select "Open in Photo Editor Pro"
3) Click on the "Tone" icon
4) Click on "Spot color". This will change the color to B&W. (Unfortunately you can't save the changes at this point.)
5) Click somewhere to select that color.
6) Click the eye dropper with the minus sign and click the same spot you just clicked to remove that color.
7) Then click the checkmark and then save it.
Ecm's suggestion above of the filters is probably easier.