I wanted to grab a colour from an image today but didn’t want to install a package so I wrote my own, thought I’d share in case anyone else is facing a similar dilemma 
/* pixcol.c - print rgb or hex colour of the pixel clicked to stdout
* This software is provided free of charge and WITHOUT warranty.
*
* Written by Nathaniel Maia, December 2018
*
* Compile with:
*
* gcc pixcol.c -lX11 -o pixcol
*
* To have output copied to the clipboard:
*
* pixcol | xclip -i -selection clipboard
*/
#include <stdio.h>
#include <stdlib.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <X11/cursorfont.h>
static XEvent ev;
static XColor col;
static Cursor cur;
static Window root;
static XImage *img;
static Display *dpy;
static int hexout = 1;
void grab_pixcol(void)
{
cur = XCreateFontCursor(dpy, XC_crosshair);
if (XGrabPointer(dpy, root, False, ButtonPressMask,
GrabModeAsync, GrabModeAsync,
None, cur, CurrentTime) != GrabSuccess) {
fprintf(stderr, "error: cannot grab the root pointer\n");
exit(1);
}
XNextEvent(dpy, &ev);
img = XGetImage(dpy, root, ev.xbutton.x_root, ev.xbutton.y_root,
1, 1, AllPlanes, XYPixmap);
col.pixel = XGetPixel(img, 0, 0);
XQueryColor(dpy, DefaultColormap(dpy, DefaultScreen(dpy)), &col);
if (hexout)
fprintf(stdout, "#%06X\n",
(((col.red / 256) & 0xff) << 16) |
(((col.green / 256) & 0xff) << 8) |
((col.blue / 256) & 0xff));
else
fprintf(stdout, "rgb(%d, %d, %d)\n",
col.red / 256,
col.green / 256,
col.blue / 256);
XUngrabPointer(dpy, CurrentTime);
XFreeCursor(dpy, cur);
XFree(img);
}
int main(int argc, char *argv[])
{
for (int i = 1; i < argc; i++) {
if (argv[i][0] == '-' && argv[i][1] == 'r' && argv[i][2] == '\0') {
hexout = 0;
} else if (argv[i][0] == '-' && argv[i][1] == 'h' && argv[i][2] == '\0') {
fprintf(stdout, "usage: pixcol [-r]\n");
return 0;
} else {
fprintf(stderr, "error: invalid option '%s'\n", argv[i]);
return 1;
}
}
if (!(dpy = XOpenDisplay(NULL))) {
fprintf(stderr, "error: failed to open display\n");
return 1;
}
root = RootWindow(dpy, DefaultScreen(dpy));
grab_pixcol();
XCloseDisplay(dpy);
return 0;
}