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.
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.