So we are talking about this:
https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-core/src/main/resources/Common/MatDefs/ShaderNodes/Basic/colorToGrey.frag

It should extract the luminance of a color (or in other words: convert RGB to Greyscale).

I think the way it has been done only works when r == g == b and then dividing by 3 again (because currently grey can be everything between 0 and 3. So it would only work when only one channel is used).

Also, when r == g == b and we'd do (r + g+ b) / 3, we could also take r, as simple as that.

So: For images already being Greyscale in RGB, we can just take one component.
For Images not being Greyscale, we need a more complicated code like https://github.com/flixel-gdx/flixel-gdx/blob/master/flixel-core/src/org/flixel/data/shaders/blend/luminosity.glsl

Luminance is only Lines 15-21 and we can add a proper RGB->HSL conversion as well.
The license is BSD.

Tagging @neph1 as he contributed the initial ColorToGrey implementation.