The channel mixer tool allows the user to control how much each of the three color channels (red, green and blue) contribute to the final grayscale brightness. It is undoubtedly one of the most powerful black and white conversion methods, however it may take some time to master since there are many parameters which require simultaneous adjustment.
Open this tool by clicking on Image > Adjustments > Channel Mixer in Adobe Photoshop. GIMP and many other image editing programs also offer this tool, however its menu location may vary.
Be sure to first click on the lower left tick box entitled "Monochrome" for black and white conversion.It is often best to get a feel for the distribution of each color channel by first setting each of the color channels to 100% individually.
Then adjust each of the red, green and blue sliders to produce an image to your liking. For an even more pronounced effect, some colors can even have negative percentages.
The sum of the red, green, and blue percentages need to equal 100% in order to maintain roughly constant brightness, although overall brightness can also be adjusted by using the "Constant" slider at the bottom. If the aim is to mimic the luminosity perceived by the human eye, set: red=30%, green=59% and blue=11%.