I've seen that as well. To me they're a product of their environment and will change depending on what they eat - perhaps they also change to look similar to their surroundings like a defensive camouflage... what threw me off is that you said other of your shrimps aren't like that so maybe they'll begin changing like that I'm not sure.
I was at my LFS about a month ago, and they had just received a shipment of 500 amanos that day. They were in a quarantine tank. They all looked like blue pearl shrimp. I had to ask if they were sick, and the manager said they always come in that color, and turn their normal brown to clear/ yellow in a couple days.
I don't know if it is from stress or food for sure but I believe it is most likely from food. I used to have a few fish in with my Amanos and I often fed my fish Hikari Micro Wafers that indeed has color enhancing ingredients. Apparently, according to Hikari, Spirulina and Astaxanthin promote color intensity. My amano's would always get some of the wafers that would sink to the boottom. During this period my amano's would turn blue.
I no longer use this food because I no longer have fish. And my Amanos have lost the
Blue in them.