Those are scales. the highlighted notes (aka the guide tones) are notes in that specific chord that give a harmonic pull towards the next chord. generally they are the 3rd and 7th note (that’s what determines what type of chord it may be: major, minor, major-minor, or dominant). they are the most harmonically important notes in the chord since they determine the quality of the chord itself, so make sure they are in perfect tune.
The answer you have highlighted in the photo is correct
Answer:
It makes this old man and his surroundings grey. This creates contrast.
