I think some tend to stay darker. The FBT I've had for over six years tends to stay light and bright though his bright green does darken slightly at times. Another FBT I got about 4 years ago was so dark you could barely make out the typical pattern you expect on FBT's backs. At one time he almost lightened up to the same color as the others, but his color changes the most. Sometimes very dark and sometimes light and bright.
So I wouldn't attribute any thing about their health based on color. I've sometimes thought that water alkalinity and lighting played a roll in when mine changed colors, but I think that was just anecdotal.
If they eat and look otherwise healthy, don't worry. The color changes of the individual FBT's may have more to do with their parentage or early environmental factors.
@Froggie85.... As for your FBT that has the "spasms" If it's not that he is shedding and trying to shake the skin loose, then I've no idea.