I’m in the process of switching from training for cycling events to training for a marathon. My intention is to still include some workouts mainly for low-impact endurance and/or recovery. For the last few weeks on a Wednesday I’ve done a short run followed by leg day at the gym and TR has suggested the Thursday is a yellow day. It has, however, left the planned endurance cycling workout as it was in the plan. Surely adaptive training should “adapt” it to the yellow day flag? Or am I missing something? It’s a first world problem but I would like to understand why. Thanks
Yellow day is rest or endurance ride. What would you like it to adapt to?
Maybe I’m misunderstanding the point of the warning then - but presumably something less intense than the previously proscribed workout. If it really thinks I should rest then why not remove the workout? Or drop the duration or the “zone”. Otherwise, if it thinks the workout is still appropriate what’s the point of the warning?
Because not everyone follows only the prescribed workouts. A lot of people will go out for an extra ride, tack on extra intensity, or go to the gym in an unscheduled way. I agree if you strictly follow the plan, then the warning isn’t necessary as the prescribed workout will already be appropriate, but not everyone does that
It’s okay to train on a yellow day, just avoid intensity as the banner in your images shows.
Red days are the ones you want to take completely off. 
Good luck with marathon training! I used TR to help train for my first marathon last spring. 
It is a good warning, means you are putting in some good work and that on that day you’d want to look at upcoming workload and how you feel. Maybe consider a slightly easier endurance ride, but generally I’m sure you’ll find that a Zn2 / Easy ride will be just fine and leave you feeling good for a run the next day!
If however it was a planned 3hr+ Endurance ride with some strong run workouts in the coming days… maybe downgrade to 2hr/1hr or more of it zone1 .
Also as a Marathon runner - I have found a lot of success using strong bike workouts to supplement a mostly easier paced running plan. Good for leg strength plus essentially the same Lactate/Mitochondria/Vo2 benefits. Like right now I’m hammering a TR plan for a gravel race in August… my bike FTP & Vo2max keep going up… just easy running, but my run vo2max is also steadily increasing (3x ez 40’ runs a week and a 2~3hr ez run (pretty year round for me))