Am curious myself as well. Been growing out the two fire eels I have had since they were small earthworm size since last August. One has remained light in color its markings/ red spotting has increased while other one has remained dark in color, markings remained the same. They are hitting the 9" mark or more I cant really measure them due to nonstop movements not staying still straight when I'm in front of their tank would say they are at least an inch high and half inch fat
When they around 5-6" which is mostly what they ban be found at, the colors will be very bright and vibrant (unless stressed, and some just have worse color than others, just luck)
They will remain with the high contrast colors for a long time, and even when they are huge they will still have them, but the colored bands do not increase in size, ratherthe fire eel will just grow biggerwhilst the color bands stay the same (not exactly how it works, but it's an easy way to explain what to expect.
I could be wrong, but either way, you do not have to be concerned about their colors becoming less noticeable until they are HUUUGE, years later