The discoloration is just due to being exposed to the air, because that part of the roots are continuously exposed to the air, and there is probably a lack of humidity in the tube that would keep that part of the root moist. The rockwool shouldn't be dry, but it shouldn't be soaked either, just moist. The roots are not wicking up enough water to keep it moist. If the plants are doing OK, then I don't know if there is any real problem. But If you want to try and lower the rockwool cubes that would help get more moisture to that part of the root system, and hopefully wick up some moisture into the rockwool cubes.
Going by the pictures, I think the easiest way to lower the rockwool cubes is to simply use the same white cups you are using for the other plants, then cut holes in the bottoms of them to place the rockwool cubes into. Personally I would experiment and make some low enough that the cubes just touched the water, and some that were just above the water level. I have seen setups that the cubes were sitting on the bottom (in the flow of water), but that will constantly saturate the rockwool cubes with water. But either way they will probably do fine. Now that I think about it, I don't think lettuce plants mind wet feet much.
P.S. Thanks for the info on the tubing, they looked square in the pictures. The rectangular rain gutter downspouts are common and easy to get here also, although for the design I have in mind I need true square 4x4 tubing or it wont work. I have found square rain gutter downspout tubing, but unfortunately not 4 inch wide.
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