Vectorise or make this code run faster

How would I make this code run faster:

for i=1:nRows
    for j=1:nCols
        Z=I(i,j,1)-I(i,j,2)-I(i,j,3);
        end
    end

Thank you

Answer 1

[Guillaume]

This is really basic matlab

Z = I(:, :, 1) - I(:, :, 2) - I(:, :, 3);