If the RGB values of an image are desaturated (color removed) the values of each are the same or nearly the same for each color, resulting in a neutral gray shade.
Desaturation pushes the Red, Green, and Blue hues towards gray. The image is still in the RGB colorspace but the colors turn gray.While desaturation results in an image that appears to be grayscale, it's not.
Corel Photo-Paint: Image > Adjust > Desaturate
Adobe Photoshop: Image > Adjust > Desaturate
Adobe Photoshop Elements: Enhance > Adjust Color > Remove Color
Jasc Paint Shop Pro: Hue/Saturation > Set Lightness to "0" > Set Saturation to "-100"