First question, does the res or the roots smell like a stale pond would?
What are your EC and PH readings telling you? Are they going up, down?
Please list all nutes and additives you have been and are using.
Regarding nutes and additives, are you running any bennies in your nute solution?
As someone has pointed out, you are running clear air line tubing... This can act almost like fibre optic cable and bounces light into your res.
Where the airline enters the bucket, start taping it up using black tape and work your way back towards the pump and cover maybe 1ft of the line in black tape.
Short of that, get it swapped out for black line ASAP.
I'm not saying it is, but the last time I saw something like that, on my plants, it turned out to be rot. I can't stress enough how many things can LOOK like rot so don't start panicking. Just watch your res temps, PH and EC closely!
My rooted clones started to look just like that.. The end of the root tips would turn brown followed more or less by the whole root mass quite quickly.
Keep a real close eye on it because if it is root rot (Pythium) then it can get out of hand in as little as 24 hours.
Some people aren't fond of this but if you're unsure, get some H202 and clean your tubs, air lines, airstones etc and even give the roots a dunking / soaking in a separate weak H202 solution while you clean and sterilize all your equipment.
If it turns out to be rot, I suggest cutting out all feed and just giving them water mixed with a weak H202 solution with some regenaroot and wiltguard. These two products are like a first aid kit for plants IMO. Pulled my girl through even after cutting the roots back to the point she had to root over again.
As I say, don't worry, it might be something as simple as staining but by all means keep a good eye on it.