Okay okay okay. I got up to 10.7 million unique colors in a 67 MP raster. I expect at least about 30% overlap because the vertical gradient wastes 25% of space and the rightmost nodes of the horizontal gradient are mostly replicated in other places. As for why it doesn't have full coverage and why there are so many repeated colors: idk yet and have to stop for today.

  <svg width="4096" height="16384" version="1.1" viewBox="0 0 4096 16384" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
   <defs>
    <linearGradient id="a" x2="4096" y1="8192" y2="8192" gradientUnits="userSpaceOnUse">
     <stop stop-color="#f00" offset="0"/>
     <stop stop-color="#ff0" offset=".125"/>
     <stop stop-color="#0f0" offset=".25"/>
     <stop stop-color="#0ff" offset=".375"/>
     <stop stop-color="#00f" offset=".5"/>
     <stop stop-color="#f0f" offset=".625"/>
     <stop stop-color="#f00" offset=".75"/>
     <stop stop-color="#000" offset=".875"/>
     <stop stop-color="#fff" offset="1"/>
    </linearGradient>
    <linearGradient id="b" x1="2048" x2="2048" y2="16384" gradientUnits="userSpaceOnUse" xlink:href="#a">
     <stop stop-color="#808080" offset="0"/>
     <stop stop-color="#808080" stop-opacity="0" offset=".25"/>
     <stop stop-color="#fff" offset=".5"/>
     <stop stop-color="#fff" stop-opacity="0" offset=".75"/>
     <stop offset="1"/>
    </linearGradient>
   </defs>
   <rect width="4096" height="16384" fill="url(#a)" stroke-width="24" style="mix-blend-mode:normal"/>
   <rect width="4096" height="16384" fill="url(#b)" stroke-width="24" style="mix-blend-mode:normal"/>
  </svg>