Hi and welcome to the forum
Fish usually show better colours when they reach sexual maturity. They also continue to eat normally.
Fish that are stressed out will go darker and stop eating or reduce their food intake. They might also act nervous and skittish.
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What are the ammonia, nitrite, nitrate & pH levels in the water (in numbers please)?
How often do you do water changes and how much water do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the tank?
How often do you clean the filter and how do you clean it?
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Any chance of a picture and short 20 second video of the fish?
If the pictures are too big for the website, set the camera's resolution to its lowest setting and take some more. The lower resolution will make the images smaller and they should fit on this website. Check the pictures on your pc and find a couple that are clear and show the problem, and post them here. Make sure you turn the camera's resolution back up after you have taken the pics otherwise all your pictures will be small.
If the video is too big for this website, post it on YouTube and copy & paste the link here. We can view it at YouTube. If you are using a mobile phone to take the video, have the phone horizontal so the video takes up the entire screen. If you have the phone vertical, you get video in the middle and black on either side.
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Do a 75% water change and gravel clean the substrate every day for a week. Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Use clean buckets that have not had anything in them except clean water.
Clean the filter if it hasn't been done in the last 2 weeks. wash filter materials in a bucket of tank water.
Increase aeration/ surface turbulence to maximise oxygen levels.