[newimage, newmap] = rgb2ind(existingimage, 32);
But it is not clear in your question whether you just want to display the unique colors (if so then display newmap) or if you want to display the indices (if so then display newimage).
I am also not sure what order you want to display the output in. The indices would be a two dimensional array, one index per pixel. You could reshape() that as a column, but will the user understand? The colormap would be an array with three columns (R, G, B), and displaying in one column does not strike me as being a good thing ?
