How do I add a legend to a boxplot in MATLAB?

       figure; 
       colors = [1 0 0; 1 0 0; 0 0 1; 0 0.5 0; 0 0.5 0; 0 0.5 0]; 
       x = boxplot(rand(100,6), 'Colors',colors); 

       % findall is used to find all the graphics objects with tag "box", i.e. the box plot
       hLegend = legend(findall(gca,'Tag','Box'), {'Group A','Group B','Group C'});

Depending on the number of variables that you have, you might not obtain the right color for each legend element. Use the following trick to manually change the color of each legend element:

       % Among the children of the legend, find the line elements
       hChildren = findall(get(hLegend,'Children'), 'Type','Line');

       % Set the horizontal lines to the right colors
       set(hChildren(6),'Color',[1 0 0])
       set(hChildren(4),'Color',[0 0 1])
       set(hChildren(2),'Color',[0 0.5 0])