How to get color of pixel in coordinates 123, 456 in screen?
I found a way to do this using only built-in components:

do shell script "screencapture -R123,456,1,1 -t bmp $TMPDIR/test.bmp &amp;&amp; \
                 xxd -p -l 3 -s 54 $TMPDIR/test.bmp | \
                 sed 's/\\(..\\)\\(..\\)\\(..\\)/\\3\\2\\1/'"


Output is the color in hex, for example, ffffff for white.

The screencapture command takes a 1x1 screenshot of the coordinates, and outputs it in the bmp format to a temporary location. 

The xxd command reads 3 bytes from that file at offset 54. 54 is the length of the header in the generated bmp file, and the next 3 bytes are the color of the pixel.

The sed command swaps the first two characters of the output with the last two. This is because the bmp stores the colors in reverse of the normal order - BBGGRR. This command swaps the order to the normal RRGGBB.