I was fortunate to attend one of Dr. Santos' lectures on these shrimp this spring. I also consulted with him by email for assistance with a research paper on these shrimp.
I summarized some of the interesting points in the lecture in this thread:
http://www.petshrimp.com/discussions/vi ... highlight=
I do recall that he mentioned that there is variability in the coloration of the shrimp in the wild.
His most current paper focuses on studying the genetic differences of these shrimp in different location on different islands of Hawaii. Surprisingly, the genetic differences found are very significant and may even suggest a future division into multiple species based on his findings.
Coloration of the shrimp is due to genetic variation but there are also environmental factors. As you stated, the dark line that indicates a full intestine would help to make the shrimp appear darker.
In the first photo, you can see that the shrimp are variable in color when photographed close up. The second photo may show seemingly darker colored shrimp but could be a result of lighting, lower magnification, and the incredible density of shrimp in the photo.
In short, if your shrimp are happy and eating, they are probably healthy even if they are not the darkest red shrimp to be found.