MATLAB: Changing the colour of lines on a graph

colorgraph

Try this:

if i == 6
    plot(x,y,'r', 'MarkerSize', 10, 'LineWidth', 2); % Plot marker in red.
else
    plot(x,y,'k');
end

subplot(2,2,[1 3]);
bar(Mean0); hold on; bar(Mean08, 'r');

Use a semicolon instead of a comma. The variable 'Eng_Strain' is a column vector so you need to vertically concatenate the zero with the vector.

Eng_Stress = [zeros(1); Eng_Stress];    
%                     ^

Or just

Eng_Stress = [0; Eng_Stress];