so... I hope im not to late, but I wanted to get in on this too.
So from what I was reading and what I grasped was that Tsugumo has his selout with using darker shades than those in the sprite. Helm was saying to use lighter shades.
So that got me to thinking. Why not combine both?
Heres a small example I through together in 5mins just for explanation purposes.

The first image shows the sphere. I have 3 colours used in it, and then one darker and one lighter for the outline. I figured that why not colour the lines like they were the sprite themself. Use the darkest shade where the shadows would be on the sprite, and the highlight colour thats closest to the lightsource. Then select one of the colours you used in the sprite itself as a transition colour. For me I used the darkest colour. BUT, the colour you used for the transition colour, CANNOT touch itself within the image. You used it as an outline therefore if it touches itself it looses some of that outline crisp, like the second image.
Now the second image, I just edited my display pic just because its the closest thing i have to a sprite that could be used in a game. Here I have only one new colour. Thats the darkest shade. I used it the same was as I did in the first. I put it in the darkest places. Next I used the colours within and extended them to the lines. I find it doesnt look as crisp, but it can still get the job done. Perhaps I even went too far and added a new darkshade for each set of colours. I could of very easily done this using one one new colour. But owell.
I just thought Id show my thought on the subject. Perhaps a solution or another viewpoint to this that helm and tsu have created. Hope it fits.