365 x 466 x 3 is already RGB, unless it is HSV or L*a*b* or similar.
If you had a 365 x 466 indexed image and a colormap then you could use ind2rgb() to convert it to RGB, but you would typically assign the result to an entire variable rather than to a portion of a variable. But it could be done, such as
for K = 1 : length(filenames)
[ind, cmap] = imread( filenames{K} );
all_images(:,:,:,K) = ind2rgb(ind, cmap);
end
