You must know that desaturating an image in Photoshop is very simple. Just press CTRL+SHIFT+U.
But should you want it to change it into a mono color of your choice, what do you do? It is also quite simple.
I will use this free stock image. You will find it here
It belongs to =faestock
Step 1 : There is only a single Background Layer in Photoshop.
Step 2 : Click the New Fill or Adjustment Layer button. Click Solid Color from the popup.
Step 3 : As soon as you click the image turns Black. The Pick a Solid Color dialogue box opens.
Step 4 : Click on Color Libraries.
Step 5 : The Color Libraries open.
Step 6 : Click the Brown in the Spectrum in the middle.
Step 7 : I have clicked on Pantone 448. Click OK.
Step 8 : The image is covered with the color as soon as you click OK.
Step 9 : Change the Blend Mode to Color.
And here is the mono color image. You can choose any color in the spectrum to pick a different color. The steps will be the same.
Other photoshop tutorials are here