Can I regard the grayscale of a RGB color image as a panchromatic image?
The workflow of fusing RGB true color image and panchromatic grayscale image can help understand this question. After converting RGB color space to the IHS space, the intensity component is replaced with the (high-resolutional) panchromatic image, and then the new intensity, hue, saturation combination as IHS are converted back to the original RGB space.