snipplets.dev/snipplets/code/C/rgbToHex.c

#include <stdio.h>

int main(int argc, char const *argv[])
{
    int red = 0;
    int green = 128;
    int blue = 64;

    printf("#%.2x%.2x%.2x\n", red, green, blue); //#008040

    return 0;
}