To get a video stream from ARCore requires drawing the texture into a OpenGL 3.1 Context what is being captured. You can find here how to do it.
After you've captured a video stream, the simplest way to convert colour video
/image
into grayscale one (the average of red, green, and blue) is to use a Colour Matrix
with zero saturation.
Here is an excerpt of Java code how to do it programmatically:
ColorMatrix BW_matrix = new ColorMatrix();
BW_matrix.setSaturation(0);
ColorMatrixColorFilter BW_filter = new ColorMatrixColorFilter(BW_matrix);
imageview.setColorFilter(BW_filter);
You need to apply it only to video stream from phone's camera, not to the rendered result after adding Sceneform's 3D models to the scene.