How to determine if changing a foreground pixel color to a background pixel color creates an unconnected line in an image.

For this problem I started by trying to using impixel to get pixel values into a matrix and when one pixel is changed from a forground to a background color they would equal and you could see the value of the line become broken. However I am finding this to be an incorrect approach to this program. I started with this small piece of code but if there is a better way of doing this than please by all means correct and help me.

%Read image into workspace 
RGB = imread('christmas tree.jpeg');
%Determine column c and row r indicies of pixel extract
c = [1 14 146 200];
r = [1 22 156 100];
%Return the data at the selected pixel locations
pixels = impixel(RGB,c,r)