The plants look great. Some species will turn color on their tops in regards to the amount of light being received, the extremes in temperatures from day to night, changes in water quality, or physiological changes related to reproduction.
If this is an old colony you may want to completely drain out any water in it and flood it with steam distilled water as with old colonies in which water is allowed to settle some salt accumulation can occur. Natural bogs are constantly recharging the water basin that the mosses sit in so that salts have no chance of burning the plants.
Check the link to see a picture of a natural colony that is white on top.

Here is a link that might be useful: {{gwi:616181}}