For a good black and white picture is one which has good contrasts (global and local) and reveals lots of textures, wherever these matter to the composition in the picture.
Now this can contribute to a better colour picture if we can extract these contrasts and textures for the colour picture.
And this can be done rather easily.
Simply make a copy of the black and white picture as a layer on top of your colour picture.
Then switch the blending mode to Luminosity and adjust Opacity to taste.
And you can try other blending modes too, like Soft Light, or Multiply or Screen.