Mine definitely change colors over time. Very similar to how they sometimes have bulbs on the tips and other times they disappear and have long tentacles. It was common for mine to have bulbs on one side of the disk and long tentacles on the other. As the tentacles change shape, the colors change -- I equate it to blowing up a balloon -- as it stretches the color changes.
Lighting also impacts the color of the anemone. As you know, bleached anemones are often a light pink color and darken with more intense light. As to how dark they get, that depends on the morph -- I've seen bright orange/red ones and also blood red ones and everything in between, mixed with green and purple.
In terms of keeping the color consistent, I don't think it's possible -- too many factors to deal with. Aside from lighting and flow (which I think plays a part in the overall shape of the nem) you also have to consider water quality -- which indirectly affects how much lighting your anemone gets (clear tank = better light penetration).
One last thing -- some people believe that feeding nems certain food can also affect their color (like color enhancing chemicals in fish food). I used to feed my RBTA salmon since I was told that it would help "brighten" up the colors -- I don't really know how true this is, but I know it didn't hurt.
