Blend black layer in 'color' mode

This page is part of the 'Converting Colour to Black and White' feature at Northlight Images

Create a new layer and fill it with black.

Change the blending mode to 'Color' and the image goes to black and white

new layer

Flatten when done to get your greyscale image.

As with many of the methods here, the variations may be quite subtle, so try it with images of your own to spot the differences.