NetLogo 7.0.0-beta2:

bitmap:channel

bitmap:channel image channel

Extracts either the alpha, red, green, or blue channel from an image. The input channel should be an integer 0-3 indicating the channel to remove (alpha=0, red=1, green=2, blue=3). The resulting image is a grayscale image representing specified channel.

Take me to the full Bitmap Extension Dictionary