std::vector<int> colors(img.x*img.y);
for(i = 0; i < img.x*img.y; ++i)
   colors[i] = *(int*)&img.colors[i];

colors = colors.sort.unique()//whatever

struct rgbega{
   int rgb;
   byte ega;
   rgbega(int rgb, byte ega):rgb(rgb), ega)ega){}
   bool operator<(int other) {
      return rgb < other;
   }
};

std::vector<rgbega> colorLUT(colors.size());

for(i = 0; i < colors.size(); ++i)
   colorLUT[i] = rgbega(colors[i], closestEGA(colors[i]));


byte getEGAFromRGB(Color _c) {
   int c = *(int*)&_c;
   return std::lower_bound(begin(colorLUT), end(colorLUT), c)->ega;
}