Verified answer

Many different ways of course, since this is Photoshop.

If the reds are just in a few places, you could try this method:

1. Add a Hue/Saturation adjustment layer on top of the original color image before it's black and white. Do this by clicking Layer/New Adjustment Layer/Hue-Saturation in the top toolbar.

2. Lower the Saturation to -100 in the Hue-Saturation box to make the image black and white. Click OK.

3. In the Layers panel click the Layer Mask Thumbnail in the Hue-Saturation layer.

4. Use the Brush Tool (type B) to brush over the red areas.

If the red areas are all over the place try this instead:

1. Use the Magic Wand Tool (type W) and click on a red area.

2. Adjust the Tolerance in the upper toolbar to get all of the reds. Make sure the Contiguous option is unchecked.

3. Select/Inverse.

4. Add an Adjustment Layer (click the black/white ball at the bottom of the Layers Panel) to make everything black and white except the red. You can Use Hue/Saturation or if you have a new version of Photoshop, just use the Black & White Adjustment Layer.

Make a selection around whatever is red in the photo and then make the photo black and white. Everything except your red selection should be black and white.

if you changed the Color Mode of the file (Image->Mode->Grayscale) then you have to change the mode to either CMYK or RGB or LAB etc. in order to start adding color back into the file