Al,
I think that the curd is yellow and whitens up as it dries. I found no less than 14 places in the video where the color of the wet curd can be compared with white objects. The curd appears yellow against white vats, aprons, hats, etc. Even the hosed on plops appear yellow (see the video at the 2:42 mark). I shoot a lot of video and won't argue that white objects can appear yellow in different lighting, backgrounds, etc. My point here is that the wet curd is consistently yellowish against many expected white objects. This discounts lighting as a source of yellow. It may well be that the milk itself is yellowish. It likely comes from Simmental cows, a rare breed here in the US, but common in Switzerland. You are also correct that other videos show Belper Knolle as having a bright white paste - at least when fully dry. So do many photographs. Curiously, those on the NEC website show a browish-gray curd against a white plate at the bottom of the BK recipe.
The first batches of cheese I made a year ago were Feta. The first batch was made with P&H milk. The drained curd was white and after a year of aging the paste is still bright white. The second batch made two weeks later used our local Twin Brooks whole milk (Jersey cows - vat pasteurized, non-homogenized). It was white coming out of the bottle, made a slightly yellow drained curd and now has a well developed yellow cast. So, color likely is more an indicator of the milk source than anything else. I have no idea how Simmental cow milk behaves from a color standpoint. Maybe this explains the color change, if there is indeed one.
But this is missing the forest for the trees. Unless I am not seeing something, the process shown in the video is not a lactic process for the simple reason that a lactic process does not create a curd with the strength of the cut curd shown in the video. Belper Knolle may have been made using a lactic process initially but it does not seem to be made this way any longer. Keep in mind that according to the recipe on the NEC website that this is a newly created cheese (maybe 15 years old). I've corresponded with Jim Wallace of NEC about BK. Jim said that he visited the BK creamery when the BK was made using a lactic process and I'll take his word that it was once made this way. Jim did not explain how the video could be reconciled with a lactic process.
Perhaps only Swiss cows can yield milk that through a lactic process will make a curd with the strength of those in the video. Jersey cows are certainly not able to do so. It would be interesting for one of our European members to make Belper Knolle using Simmental cow milk using a lactic process. My next batch will be to go the rennet route, cut the curd at the appropriate time, drain when firm enough not to shatter and stack it in a vat until the pH drops to about 4.6 and finish as in the video. I'll report the results in a new thread.
Update: 02/14/16:
Last night I had a flash of inspiration and I think I now know how the cheese in the video is made. I want to "prove" this is the method but won't get time to do it for a couple of more days. Sorry for hijacking this thread. Back to you, Al!
Kern