Grading Colour To Black And White In Davinci Resolve
There is actually a specific option for this in Resolve.


In your primaries pane (bottom left of the color tab) navigate to the "rgb mixer" tab.
In the top right of the tab, click the options menu (3 dots)
Check the 'make monochrome' command.


Now you'll see 6 from the 9 rgb mixer bars turn gray and you'll be able to use the other 3 to mix you black and white the way you want to. 

Good luck!