script to answer Quora question

colortest.c

/* http://www.quora.com/Color-and-Colors/Are-there-any-colors-other-than-black-for-which-the-RGB-values-are-the-same-as-the-HSV-values */

#include <stdio.h>
#include <stdbool.h>

float MIN(float a, float b, float c) {
    if (a <= b && a <= c) return a;
    if (b <= a && b <= c) return b;
    return c;
}

float MAX(float a, float b, float c) {
    if (a >= b && a >= c) return a;
    if (b >= a && b >= c) return b;
    return c;
}

/* r,g,b values are from 0 to 1
 * h = [0,360], s = [0,1], v = [0,1]
 *              if s == 0, then h = -1 (undefined)
 *
 * adapted from:
 * http://www.cs.rit.edu/~ncs/color/t_convert.html#RGB%20to%20HSV%20&%20HSV%20to%20RGB
 */
void rgb2hsv(float r, float g, float b, float *h, float *s, float *v) {
    float min, max, delta;
    min = MIN( r, g, b );
    max = MAX( r, g, b );
    *v = max;
    delta = max - min;
    if (max != 0) {
        *s = delta / max;
    } else {
        /* r = g = b = 0  =>  s = 0, v is undefined */
        *s = 0;
        *h = -1;
        return;
    }
    if (r == max) {
        /* between yellow & magenta */
        *h = ( g - b ) / delta;
    } else if (g == max) {
        /* between cyan and yellow */
        *h = 2 + ( b - r ) / delta;
    } else {
        /* between magenta and cyan */
        *h = 4 + ( r - g ) / delta;
    }
    *h *= 60; /* convert to degrees */
    if( *h < 0 ) {
        *h += 360;
    }
}

inline bool inteq(float x, float y) {
    return (int) (x * 255) == (int) (y * 255);
}
inline bool eq(float x, float y) {
    float diff = y > x ? (y - x) : (x - y);
    return diff < 0.0035; /* epsilon < 1/255 */
}

bool same(float r, float g, float b) {
    static float fact = 1 / (float) 360;
    float h, s, v;
    rgb2hsv(r, g, b, &h, &s, &v);
    h *= fact;
    /* use `eq' for 0.0-1.0, or `inteq' for 0-255 */
    if (inteq(r, h) && inteq(g, s) && inteq(b, v)) {
        return true;
    }
}

void main() {
    int i, j, k;
    int count = 0;
    float fact = 1 / (float) 255;
    printf("Starting search...\n");
    for (i = 0; i < 255; i++) {
        for (j = 0; j < 255; j++) {
            for (k = 0; k < 255; k++) {
                float r = i * fact;
                float g = j * fact;
                float b = k * fact;
                if (same(r, g, b)) {
                    printf("Found a pair: (%d, %d, %d)\n", i, j, k);
                    count++;
                }
            }
        }
    }
    printf("Done. %d pair(s) found.\n", count);
}