The graythresh function in MATLAB is used to determine the threshold level for a grayscale image, but it cannot be used directly on an indexed image. To use graythresh on an indexed image, you need to first convert the indexed image to a grayscale image.

Here's an example of how to use graythresh on an indexed image in MATLAB: 

% Load an indexed image
[X,map] = imread('example.tif');

% Convert the indexed image to a grayscale image
I = ind2gray(X,map);

% Use graythresh to determine the threshold level
level = graythresh(I);

% Convert the grayscale image to a binary image using the threshold level
BW = imbinarize(I,level);

% Display the binary image
imshow(BW)

In this example, the ind2gray function is used to convert the indexed image X and its colormap map to a grayscale image I. Then, graythresh is used to determine the threshold level for the grayscale image I. Finally, imbinarize is used to convert the grayscale image I to a binary image BW using the threshold level.

https://matlabhelpers.com/matlab-homework-help.php