I don't think that they are less hardy then other color. When we buy these hard to find these colors, we tend to have lower threshold to buy, thus we often buy sicker animals. I am guilty of this myself. They are not harder to keep IMO.
I finally got a Blue Carpet three weeks ago. It is a small one only about 5 inches or so. It is in my 24 g tank with my green carpet that I have had for 20 months. I feed the green one only sparingly to keep him small in my office tank. The tank olso have two Onyx Percula, two Crocea clams, various small SPS frags and Xenia. I chage 6 g water every week. Everything is doing well and stable for the last year.
My Blue Carpet is not dong well. It is deflating every afternoon and back to full inflation every morning when I come into my office. This is going on for three weeks now. Yesterday, it was fully inflated at 6PM for the first time. I hope that this is the first sign that it is doing well and turn the corner. I have not try to feed it yet other than fish food, which stick to the anemone but the two clowns just pick them off after a few seconds.
Anybody with Haddoni start out like this but pull through and do well? BTW, Salinity is 35 ppt and temperature is on the low side at 74 AM, 77 PM. Light 150 W DE 10000K MH on 12 hrs per day.
I have not test any paramenter for a long time but IME, this much water change and this level of load (two fish) all the water parameter should be well within tolerance of Haddoni (plus my older Haddoni, clams and coral all really doing well.)