If you want to try something quick and free, download the following software
at:
www.irfanview.com
First, make a copy of your color photo. Right click on the photo and click
on Copy on the opening menu. Then, click on File on your top bar and click
on Paste on the opening menu. A new file will be added in your folder. Same
name as the original file but with the words "Copy of" in front of the
original file name.
Open Irfanview and open the file Copy of........
Click on Image on the top bar and then Enhance color.
A window will open. Move the saturation slider to the left and compare the
picture on the left with the picture on the right to see how much color you
are removing. When satisfied, click on Apply to image and then save the
file. You now have a color photo file and a black and white (or gray scale)
photo file.