Some production layer breeds will get a little shabby looking when they are laying well...it seems to be a bit of a trade off for consistent, daily lay. My granny always used to say that the scruffiest looking bird in the flock will be your best layer and I would have to agree with her on that, particularly with the RIR breed or sex link birds. My best layer in this current flock is a little New Hampshire hen that just can't be beat..but she's never really glossy like the other birds and her comb doesn't get that bright, cherry red.
Nutrition will be diverted to maintaining reproduction when they are laying well and not so much into plumage...and, conversely, when a bird is molting heavily and trying to recover from it, nutrients get diverted to feather regrowth and not towards production. In a true layer breed it will mostly go into reproduction most of the time, sometimes leaving them the ugly girl at the prom.
Many newbies rush to deworm their birds at the slightest hint of dull feathering or comb, but I won't be the one to ever recommend that to you...there are several reasons for individual birds to have dull feathering or a less than cherry red comb~only one of which could be anemia caused by a severe parasite infestation, at which time they would not likely be laying well. For birds with good nutrition, good care and a naturally clean living environment, anemia from internal parasites would be the very last thing I would think of when assessing appearances. If they were being raised in horrible conditions, it would be more inclined to believe they had an infestation above other causes and would act accordingly.
I don't think much of DE as a wormer or as anything I'd use in my chicken arsenal, though it seems all the rage right now for that very thing. I'd just give it a wait and see attitude and watch how they go along, but I wouldn't be getting nervous about internal parasites with them just yet.