You can convert it to a color image and then set the pixel to red, like harjeet did. Of course I hope you know that the RGB image is no longer a binary image, so don't use that in any further processing, like pass it to regionprops() or bwconncomp() or something.
An alternative is to just plot a red dot over the pixel.
imshow(binaryImage); % Display it.
% Plot a red dot at row 30, column 173
hold on;
plot(173, 30, 'r.', 'MarkerSize', 10);
Make the marker size whatever you want. This does not create a new color image and does not change the binary image, it just plots a red dot in the overlay above the image. The two solutions so far will both make a red pixel on the displayed image at the designated position, but they do it in different ways. Pick whichever solution does it in the way you want most.
