I love it when the volume extras turn out nice. Also a chance for the author to do something a little different.
I think what's throwing people off about these color pages is probably that it's done in (i think) watercolors? So a much softer feel compared to the crisp, line art of regular chapters and color contrast of black and white with swathes of that 90s neon green (for a vibe matching the era of the music highlighted in the manga / uncle and his friends' youth period).
One more thing: I could get lost in Mitsuki's grey eyes all day...