Extract an RGB channel of an image

JavaScript (ES6), 29 bytes

a=>n=>a.map(b=>b.map(c=>c&n))

Input is a 2D array of 24-bit integers (e.g. [[0x0000ff,0x00ff00],[0xff0000,0xffffff]]) and 16711680 for red, 65280 for green, 255 for blue. If this isn't valid, try this instead:

JavaScript (ES6), 48 bytes

a=>n=>a.map(b=>b.map(c=>c.map((d,i)=>i==n?d:0)))

Input is a 3D array of color values and 0 for red, 1 for green, 2 for blue.

Mathematica, 13 bytes

ImageMultiply

This should be the legitimate version of JungHwan Min's answer. This function takes the image and one of Red, Green, Blue as input. For example:

As a fun-fact, there's also ColorSeparate which gives you individual channels, but it returns them as single-channel/greyscale images, so you'd need to multiply them by the colour afterwards anyway.

Bash + ImageMagick, 35 32 27 bytes

mogrify -channel $1 -fx 0 i

Assumes the image is in the file i and the script takes one of RG, BG, BR for blue, red, and green respectively; outputs to the file i.