"black or white" is a dot. "shades of gray" is like an axis on which you can plot points.

"color" on the other hand is like a 3-dimensional space, and so points can no longer be compared and sorted. How do you compare green with blue? You cannot.

On the topic, you can compare the rudeness of two messages and you can also establish some threshold over which the rudeness is no longer acceptable. That's why people use the "shades of gray" metaphor, because it's still useful.

And indeed, life has color, but that's why life is complicated, which is why we feel the need to simplify its dynamics.

It's a gradient, not a binary.

Adding colors to the metaphor adds words with no increase of understanding.

Understanding is not disjoint from emotion.

What is this, arguing for the sake of arguing? He answered your question perfectly. Adding color for "emotions" makes absolutely no sense in this context.

No, but using a broken metaphor confuses things. The phrase:

"Don't see things as either black or white, the world has millions of hues"

conveys the message that situations are nuanced as well as being truer to how things are with respect to color.

So, it's better than some constrained metaphor about a BW gradient (much more since lots of people don't even know what a gradient is).

The metaphor isn't broken. What you propose is broken.

The word "gradient" isn't even part of the metaphor, so I don't see how that would cause confusion. What comes between black and white? Gray. Not millions of color hues. Shades of gray makes sense and fighting to try and get colors in the mix somewhere is only serving to muddy this up.

Edit: Actually, here's another perspective: millions of color hues come between black and white only if you're thinking about colors to begin with. (Black and white aren't strictly considered colors even.) So while I can see where you're getting this from, I still think it doesn't serve anyone well to try and spice it up from the very well understood meaning of the phrase.