If I understand correctly, each sheet has the red (R) value in the first column, the green (G) value in the second and the blue (B) value in the third column. So with a 16x16 image, there should be 3 columns and 16 rows, in 16 sheets. Is that correct?
The function xlsread() can read one sheet at a time, making it possible to do a simple loop going through all the sheets, converting each one to a column.
% Make empty array for the image, this makes Matlab work a bit faster
img = zeros(16,16,3);
% Use a for loop to read each sheet, and insert it into a column in the image
for column = 1:16
img(:, column, :) = xlsread('NAME_OF_EXCEL_FILE.xlsx', column);
end
% Depending on the format of your data, you might have to convert to uint8
img = uint8(img);
I think that should do the trick, but without having your data, it is hard to test. Good luck with it :-)
