Because of the current start of ruin of the Parthenon in Athens, many theories about how it would originally have looked are somewhat speculative. The fact that Lord Elgin removed many of the sculptures, in no way helps either.
While looking for something else, I came across information on Emory University’s Parthenon Project. They were aiming to try & see how the frieze on the Parthenon might have originally looked from ground level. This fascinated me, as I spent a lot of time creating 3D models to research this same aspect of the building in 2000.
The viewpoint taken by many, is that due to its location & restricted viewing angle, the frieze would have been barely visible to people viewing the Parthenon on the Acropolis, if they did not already know about it. Even then, their views would be limited, because it would be seen from such a steep angle.
With their Parthenon Project, Emory University’s students aimed to use the replica of the Parthenon in Nashville  to test out the various theories about the visibility of the frieze.
Although Nashville’s Parthenon is a close replica of the actual Parthenon, it never had the frieze installed due to a lack of funds. This meant that the first task for the students was to recreate the frieze panels. They did this in a variety of ways, creating them flat & in relief, in colour and in black and white. This use of colour is a very interesting step. We know that the panels were originally painted, but when we visualise them, we still tend to see them as they are today in the Acropolis Museum & British Museum, where the detail on them is formed by the shadows cast & therefore becomes more visible when the light is less diffuse. What had not been tested before was how the painting on the surface of the sculptures would have helped to define them more clearly, making the fine detail far more apparent even in the comparative gloom of the location of the frieze (compared to the metopes which were in bright sunlight).
I would be interested to see this experiment re-attempted in Athens – although I’m not sure where it could be done, as the Parthenon now has no roof. The attic sunlight is breathtaking in its sharpness & I wonder whether the sculptures would still be as clear to see on a summers day there as they were in the Nashville experiment.
Visit the website for the project  for far more detail about its aims & the issues they encountered in trying to recreate what was originally there.
Emory University 
The Problem: the Visibility of the Parthenon Frieze
By Bonna D. Wescoat
The Parthenon is the most famous ancient Greek building, and its celebrated frieze, dispersed between London, Paris, and Athens, is one of the icons of western art. We view the frieze today at eye level within a museum setting, but originally it was placed at the top of the cella wall behind the surrounding colonnade. The location has baffled scholars, who find a serious disjunction between the high level of articulation and meaning, and the low level of visibility. Scholarly opinion on the visibility of the Parthenon frieze is universally negative. The frieze is described as illegible and fragmented, its position dark and cramped. Photographs tend to confirm the awkwardness of the position. In making this assessment, we are of course seriously hindered by the state of the remains. The reliefs are no longer on the building, and the building no longer has its ceiling and roof.
Scholars and the general public have long admired the precise replica of the Parthenon built in the 1920s in Nashville because it allows us to recapture some of the experience of being in an ancient Greek temple. But there is one very important way in which scholars have not yet mined the value of the Nashville Parthenon: it has the capacity to serve as a crucial tool for understanding the visibility of the Parthenon frieze.
In my many trips to the Nashville Parthenon, I have been persuaded that the area the frieze occupied was neither dark nor illegible. The human eye (or more properly, the embodied eye) has a rich sensory perception, enhanced by peripheral vision, absorption over time, and memory, that allow this space to be eminently legible, only in a different way from that of the decorated pediments and metopes on the outer part of the building.
The director of the Nashville Parthenon, Wesley Paine, and the curator of education, DeeGee Lester, have agreed to allow our seminar on Ancient Greek Architectural Decoration (11 graduate and undergraduate students), to engage in experimental archaeology to test the visibility of the Parthenon frieze in as near conditions to the original as exist. We have permission to recreate panels of the frieze and install them in situ. We will then engage the public in a series of exercises designed to gauge how well we can see the frieze in real time and space.
We made a trip to Nashville at the beginning of the semester (September 15th) to determine the kind of intervention that would be most effective. The students decided to concentrate on the west side of the building, and in particular, the NW corner. After long discussion, the consensus was that color was the most critical factor in the visibility of the frieze. We were willing to forgo relief in the interests of time, expense, and achievability. To recreate the panels, we decided that canvas would be both cost effective and far more durable than paper. We were able to make five painted panels. We will make a sixth panel in relief, using a 2-inch panel of insulation foam. The staff of the Nashville Parthenon has agreed to take down the bird netting and install the panels in their correct position on the building. We will then recreate the processional routes on the Athenian Akropolis, staking out the paths and the impediments with contractors’ spray. We will have all who attend the event move along the passages and describe how well, and how much of the frieze they can see. The “hot” spots will be marked with contractors’ flags. Observers will complete a short questionnaire, which, along with a video and still photographs, will serve as documentation for the visibility of the frieze. The staff of the Nashville Parthenon will publicize the event to help build our base of observers. We are tentatively scheduled to execute the experiment on November 9-10th.
We anticipate that this experiment could become a paradigm-shifting intervention in the studies of the Parthenon frieze.