Don't worry about it. You can use a grayscale image as an indexed image as-is. Just set up the colormap based on the grayscale and set the map for whatever colors you want for the special values.
grayImage = imread('pout.tif');
% Initialize a normal colormap for a normal gray scale image.
cmap = gray(256);
% Set up special colors for 91, 123, 255.
cmap(92, :) = [1, 1, 0]; % Gray scale 91 will show up as yellow.
cmap(124, :) = [1, 0, 1]; % Gray scale 123 will show up as magenta.
cmap(256, :) = [1, 0, 0]; % Gray scale 255 will show up as red.
imshow(grayImage, cmap);
colorbar
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0, 0.04, 1, 0.96]);
However, note that the index (row) of the colormap you want to use is one more than the gray level, because row 1 controls the color of gray level 0, row 2 controls the color of gray level 1, and so on until row 256 controls the color of gray level 255.
