Till the botanists straighten it all out, this is what I've come up with based on my own experience.
I think there's at least 4 versions of "E. tenellus." I've got all 4 of them in my 90gal, and this is consistently how they grow even under the same conditions:
1- the "regular" grows 4-6" tall and stays green no matter the light/nutrients.
2- the "narrow" grows 2-3" tall and also stays green no matter the lighting.
3- the "red" or "pink" also grows 4-6" tall, but the leaves are narrower, darker green than the "regular," and also can turn red/pink/bronze. Edward- the ones that you sent me I have planted right next to "regular," and even the new runners coming off of these plants show these traits as opposed to the well-established "regular" that are growing right next to them.
4- "micro" grows 2-3" tall, but the leaves are narrow and have the same coloration as the "red" or "pink."
Personally, I wouldn't be surprised if these need to be broken out into different species. Especially the ones that will versus will not turn pink. The leaves do look pretty dramatically different- I have no issues telling them apart in my tank.
Hopefully the research you're helping with will shed some light on all this, Daniel.