Day after day I learn new tricks on how to edit photos. What fascinates me most is the craftsmanship of doing things on hard way aka step by step and not just using some automatic GIMP scripts or PhotoShop actions. …
Very interesting thing I found by trial and error is the way to overlap a duplicate layer in colors over the black and white base layer. Although, I'm sure there are several instructions where this has been explained already.
The idea is to duplicate the base layer, then change the layer blend mode into something suitable like 'overlay'. Next thing to do is change the base layer into B&W. On PhotoShop this happens via 'Layer > New Adjustment Layer > Channel Mixer'. Then just hit OK and on next screen you can select 'Monochrome'. On GIMP this is much easier since the 'remove colors' option affects only on selected layers.
As a result you get B&W base layer that glimmers through overlapping layer displaying the same in colors. But now the colors look either washed out or even almost invisible, depending on what blend mode you choose. On some blend modes you still get strong and vivid colors, which you can then lessen by changing the layer opacity.
Finally you can play with vignetting and add new layer, but instead of using white or black vignette, you can try something else like gray-fill-in color. Then switch the layer blend mode to something else like 'soft light', cut hole into the layer, and use Gaussian blur generously to create nice gray vignette.