goldfish changes color red to white

Question
Hi, i have an orange, brown, and 2 red/white goldfish in my tank.  one of the red/white goldfish starting losing its red color and now its practically lost all of its red and is almost completely white.  the other red/white goldfish however has all of its color though.  all of my goldfish seem normal and active.  i change about 50% of the water weekly.  but i dont add anything else to the water besides dechlorinator.  ive had these fish for about... half a yr.  feed them flakes and pellets about twice a day.  florescnt lighting for lights.  if you know anything it would be helpful. thank you

Answer
When my goldfish did this some one told me that it happens when they arent getting the right nutrients in there diet, and yes it happened to only one fish with me, so i switched there food to tropical food as it has more nutrients that goldfish apparently dont need, and she got her colour back within 2 weeks.

Also it could change colour if its sick or when there isnt enough light, but all your fish would loose colour then.

The last thing would be your fish hadnt got his/her adult colour, and has just got it now, baby goldfish are born black/brown and get there colour between 6 months and 2 years depending on what there fed.

Try changing about 30% of the water rather than 50% as that 100% water change in 2 weeks and all the good bacteria will have to build up again.