Customary Method - Grayscale

In this method, we simply click Image > Mode > Grayscale in Adobe Photoshop and our image is converted to black and white.  The advantages of this method are that it is quick, intuitive and retains perceived luminance.

However, the disadvantages are many.  This method is a destructive method.  The image is literally no longer a RGB image.  Instead of three color channels, there is now only one grayscale channel.  The specific information captured in the three color channels has been permanently lost.  Some advocates say we can reconvert the image back to RGB.  However, we do not get back the original RGB channels.  We get the grayscale channel replicated into the red, green and blue channels.  This does not restore the original RGB information.  Therefore, we lose the ability to manage tone by color.  To demonstrate this to yourself, convert an image from RGB to true grayscale and back to RGB (click Image > Mode > Grayscale and then click Image > Mode > RGB Color).  Did any of the colors reappear?  No, because all color information was lost when first converted to grayscale.

Figure 1 shows the two sample images converted to true grayscale.  Figure 2 shows the same two images converted in such a manner as to retain measured luminance.  Which is more accurate?  The choice is yours.  Grayscale retains the perceived tone as defined by %K.  The Measured luminance approach retains the measured tone as defined by the Curves control.