Find most luminous contrasting color

contrast.c

#include <math.h>
#include <stdio.h>

double f(int c) {
  double x = c / 255.0;
  return x<=0.03928 ? x/12.92 : pow((x + 0.055)/1.055, 2.4);
}

double rellum(int r, int g, int b) {
  return 0.2126*f(r) +  0.7152*f(g) + 0.0722*f(b);
}

double contrast(int c[3], int r, int g, int b) {
  double l1 = rellum(c[0],c[1],c[2]), l2 = rellum(r,g,b);
  double ct = (l1+0.05)/(l2+0.05);
  return ct>1 ? ct : 1/ct;
}

int colors[6][3] = {{106, 228, 185},
                    {  0, 186, 244},
                    {114, 164, 255},
                    {247, 143, 231},
                    { 88, 221,  19},
                    {229, 240,  64}};

int main(int argc, char **argv) {
  int r0=0, g0=0, b0=0;
  double lum = 0.0;
  for (int r=0; r<=255; r++)
    for (int g=0; g<=255; g++)
      for (int b=0; b<=255; b++) {
        int good = 1;
        for (int i=0; i<6; i++)
          if (contrast(colors[i], r,g,b)<7.0) {
            good = 0;
            break;
          }
        if (good) {
          double l = rellum(r,g,b);
          if (l > lum) {
            lum = l;
            r0 = r; g0 = g; b0 = b;
          }
        }
      }
  printf("#%02x%02x%02x (%0.5f)\n", r0, g0, b0, lum);
  return 0;
}

contrast.lisp

(declaim (optimize (speed 3)))

(defun f (c)
  (declare (type (integer 0 255) c))
  (let ((x (/ c 255.0d0)))
    (if (<= x 0.03928d0)
        (/ x 12.92d0)
        (expt (/ (+ x 0.055d0) 1.055d0) 2.4d0))))

(defun rel-lum (r g b)
  (declare (type (integer 0 255) r g b))
  (+ (* 0.2126d0 (f r)) (* 0.7152d0 (f g)) (* 0.0722d0 (f b))))

(defun contrast (r1 g1 b1 r2 g2 b2)
  (let* ((l1 (rel-lum r1 g1 b1))
         (l2 (rel-lum r2 g2 b2))
         (ct (/ (+ l1 0.05d0) (+ l2 0.05d0))))
    (max ct (/ ct))))

(defparameter colors '((106 228 185)
                       (  0 186 244)
                       (114 164 255)
                       (247 143 231)
                       ( 88 221  19)
                       (229 240  64)))

(defun best-contrast (colors)
  (let ((r0 0) (g0 0) (b0 0) (lum 0.0d0))
    (dotimes (r 256)
      (dotimes (g 256)
        (dotimes (b 256)
          (when (loop for (r1 g1 b1) in colors
                      always (>= (contrast r g b r1 g1 b1) 7.0d0))
            (let ((l (rel-lum r g b)))
              (when (>= l lum)
                (setf lum l r0 r g0 g b0 b)))))))
    (values (format nil "#~2,'0x~2,'0x~2,'0x" r0 b0 g0) lum)))