See example 
Two issues with the line charts coloring: the colors are quite dark, that works well in the legend but less so with the lines themselves. The purple line comes close to black. Even more so because the line is actually a dark purple area with a black border. That may help to let a yellow line stand out against the white background. But for darker colors it brings those even closer to black.
If I may quip on this: it's a case of obfuscation by beautification. ;-) Just like 3D pie charts interfere with the actual message.