I don't think Kindle ads were available in my region in 2015 because I don't remember seeing these back then, but you're a lucky one to fix this classic mistake :-)

I remember trying out some of the home-made methods while I was implementing a creative work section for a school assignment. It’s surprising how "flat" the basic average looks until you actually respect the coefficients (usually some flavor of 0.21R + 0.72G + 0.07B). I bet it's even more apparent in a 4-bit display.