Without using Grayscale mode, change the photograph so it is black and white.

Basically you could opt  for the desaturate option from Image > Adjustments > Desaturate or the shortcut Shift + Ctrl + U

If you want a more advanced technique you could use the Channel Mixer from Image > Adjustments > Channel Mixer and on the bottom left click on Monochrome and adjust the sliders until you get a desired result.

You could also go to Image > Adjustments > Hue/Saturation and make the Saturation=(-100)

There's also Image> Adjustments > Black/White

You could also Image> Adjustments > Gradient Map and assign a black and white map

Press Key

Shift -