ColorSeparate

ColorSeparate[image] gives a list of single-channel images corresponding to each of the color channels in image.

ColorSeparate[image, colorspace] gives a list of images corresponding to the components of colorspace.

ColorSeparate[image, channel] returns a single-channel image containing the specified channel.

Examples

Separate RGB channels:

{r, g, b} = ColorSeparate[img]

Get the red channel:

ColorSeparate[img, "R"]

Please visit the official Wolfram Language Reference for more details.