What's wrong with this RGB to XYZ color space conversion algorithm?

I believe here is your problem, this is truncating to an integer:

CGFloat var_R = (*inR / 255); //R from 0 to 255CGFloat var_G = (*inG / 255); //G from 0 to 255CGFloat var_B = (*inB / 255); //B from 0 to 255

Try this:

CGFloat var_R = (*inR / 255.0f); //R from 0 to 255CGFloat var_G = (*inG / 255.0f); //G from 0 to 255CGFloat var_B = (*inB / 255.0f); //B from 0 to 255

I haven't checked the rest of the code for other problems.

*inR/255 is an integer division. 1/255 is zero. Write *inR/255.0 instead.

#include <stdio.h>#include <math.h>float ref_X = 95.047;float ref_Y = 100.0;float ref_Z = 108.883;void convertRGBtoXYZ(int inR, int inG, int inB, float * outX, float * outY, float * outZ) {    float var_R = (inR / 255.0f); //R from 0 to 255    float var_G = (inG / 255.0f); //G from 0 to 255    float var_B = (inB / 255.0f); //B from 0 to 255    if (var_R > 0.04045f)        var_R = powf(( (var_R + 0.055f) / 1.055f), 2.4f);    else         var_R = var_R / 12.92f;    if (var_G > 0.04045)        var_G = powf(( (var_G + 0.055f) / 1.055f), 2.4f);    else        var_G = var_G / 12.92f;    if (var_B > 0.04045f)        var_B = powf(( (var_B + 0.055f) / 1.055f), 2.4f);    else        var_B = var_B / 12.92f;    var_R = var_R * 100;    var_G = var_G * 100;    var_B = var_B * 100;    //Observer. = 2°, Illuminant = D65    *outX = var_R * 0.4124f + var_G * 0.3576f + var_B * 0.1805f;    *outY = var_R * 0.2126f + var_G * 0.7152f + var_B * 0.0722f;    *outZ = var_R * 0.0193f + var_G * 0.1192f + var_B * 0.9505f;}void convertXYZtoLab(float inX, float inY, float inZ, float * outL, float * outa, float * outb) {    float var_X = (inX / ref_X); //ref_X = 95.047    float var_Y = (inY / ref_Y); //ref_Y = 100.0    float var_Z = (inZ / ref_Z); //ref_Z = 108.883    if ( var_X > 0.008856 )         var_X = powf(var_X , ( 1.0f/3 ));     else         var_X = ( 7.787 * var_X ) + ( 16.0f/116 );    if ( var_Y > 0.008856 )        var_Y = powf(var_Y , ( 1.0f/3 ));     else        var_Y = ( 7.787 * var_Y ) + ( 16.0f/116 );    if ( var_Z > 0.008856 )        var_Z = powf(var_Z , ( 1.0f/3 ));     else         var_Z = ( 7.787 * var_Z ) + ( 16.0f/116 );    *outL = ( 116 * var_Y ) - 16;    *outa = 500 * ( var_X - var_Y );    *outb = 200 * ( var_Y - var_Z );}void convertLabtoXYZ( float inL, float ina, float  inb, float * outX, float * outY, float * outZ) {    float var_Y = ( inL + 16 ) / 116;    float var_X = (ina/500) + var_Y;    float var_Z = var_Y - (inb/200);    if ( powf(var_Y,3.f) > 0.008856 )         var_Y = powf(var_Y,3.f);    else        var_Y = ( var_Y - (16/116) ) / 7.787;    if ( powf(var_X,3.f) > 0.008856 )         var_X = powf(var_X,3.f);    else         var_X = ( var_X - (16/116) ) / 7.787;    if ( powf(var_Z,3.f) > 0.008856 )        var_Z = powf(var_Z,3.f);    else        var_Z = ( var_Z - (16/116) ) / 7.787;    *outX = ref_X * var_X;     //ref_X =  95.047     Observer= 2°, Illuminant= D65    *outY = ref_Y * var_Y;     //ref_Y = 100.000    *outZ = ref_Z * var_Z;     //ref_Z = 108.883}void convertXYZtoRGB(float inX, float inY, float inZ, int * outR, int * outG, int * outB) {    float var_X = inX/100;    float var_Y = inY/100;    float var_Z = inZ/100;    float var_R = var_X *  3.2406 + (var_Y * -1.5372) + var_Z * (-0.4986);    float var_G = var_X * (-0.9689) + var_Y *  1.8758 + var_Z *  0.0415;    float var_B = var_X *  0.0557 + var_Y * (-0.2040) + var_Z *  1.0570;    if ( var_R > 0.0031308 )        var_R = 1.055 * powf(var_R, ( 1.0f / 2.4 ) )  - 0.055;    else         var_R = 12.92 * var_R;    if ( var_G > 0.0031308 )         var_G = 1.055 * powf(var_G, ( 1.0f / 2.4 ) ) - 0.055;    else         var_G = 12.92 * var_G;    if ( var_B > 0.0031308 )        var_B = 1.055 * powf(var_B, ( 1.0f / 2.4 ) ) - 0.055;    else        var_B = 12.92 * var_B;    *outR = (int)(var_R * 255);    *outG = (int)(var_G * 255);    *outB = (int)(var_B * 255);}float Lab_color_difference( float inL1, float ina1, float  inb1, float inL2, float ina2, float  inb2){    return( sqrt( powf(inL1 - inL2, 2.f) + powf(ina1 - ina2, 2.f) + powf(inb1 - inb2, 2.f) ) );}float RGB_color_Lab_difference( int R1, int G1, int B1, int R2, int G2, int B2){    float x1=0,y1=0,z1=0;    float x2=0,y2=0,z2=0;    float l1=0,a1=0,b1=0;    float l2=0,a2=0,b2=0;    convertRGBtoXYZ(R1, G1, B1, &x1, &x1, &z1);    convertRGBtoXYZ(R2, G2, B2, &x2, &x2, &z2);    convertXYZtoLab(x1, y1, z1, &l1, &a1, &b1);    convertXYZtoLab(x2, y2, z2, &l2, &a2, &b2);     return( Lab_color_difference(l1 ,a1 ,b1 ,l2 ,a2 ,b2) );}void main(int argc, char const *argv[]){    int R1,G1,B1,R2,G2,B2;    float x=0.f,y=0.f,z=0.f;    float l=0.f,a=0.f,b=0.f;    R1 = 200;    G1 = 2;    B1 = 50;     R2 = 200;    G2 = 2;    B2 = 70;     printf("LAB DISTANCE = %lf \n", RGB_color_Lab_difference(R1,G1,B1,R2,G2,B2) );    /*convertRGBtoXYZ(R, G, B, &x, &y, &z);    printf("R =%d,G =%d,B =%d,x =%lf,y =%lf,z =%lf\n",R,G,B,x,y,z );    convertXYZtoLab(x, y, z, &l, &a, &b);    printf("x =%lf,y =%lf,z =%lf, l =%lf,a =%lf,b =%lf\n",x,y,z, l,a,b );    convertLabtoXYZ( l, a, b ,&x, &y, &z);    printf("x =%lf,y =%lf,z =%lf, l =%lf,a =%lf,b =%lf\n",x,y,z, l,a,b );    convertXYZtoRGB( x, y, z,&R, &G, &B);    printf("R =%d,G =%d,B =%d,x =%lf,y =%lf,z =%lf\n",R,G,B,x,y,z );*/}

Color conversions and differences in C https://github.com/gi0rikas/Color-conversions Lab distance ~= 2.3 corresponds to JND(Just noticeable difference)