The documentation for the plot command states that you can set the LineSpec immediately after X and Y value arrays, which is why this works in your loop:
plot(t,means(k,:)*10^13,color{c})
because color{c} defines a LineSpec value (in your case, a color). According to the documentation the LineSpec can be any of "line style, marker symbol, and colorstring". Notice that this list does not include line width.
If you read the only section of the documentation that mentions line widths, you will see that this occurs under the section titled "Name-Value Pair Arguments". Name-Value Pairs are often used in MATLAB to enter optional arguments for many functions. You can read more information in the plot documentation.
For the sake of completeness, this is an example that the plot documentation contains, which includes changing the line width:
x = -pi:pi/10:pi;
y = tan(sin(x)) - sin(tan(x));
figure
plot(x,y,'--gs',...
'LineWidth',2,...
'MarkerSize',10,...
'MarkerEdgeColor','b',...
'MarkerFaceColor',[0.5,0.5,0.5])
