Look for a brightly colored juvenile - there is a lot of variation in how much red they have in their tail/pectorals. Mine has stronger color than most and he was already showing it when I bought him at about 3.5-4".
Sorry to bring up an old thread, but I have the same question regarding tail color.
Do they develop a red tail as they get bigger? Or are they just different species?
Just got a 2" flagtail with bland striped but no color tail. Showing some red in fins. Any potential to have red striped tail as it gets bigger?
They are several different species. There are 2 with the red tail. One has more red in the fins other than tail. There is also a yellow one that isn't usually a bright yellow.
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About 3" onward and color will become apparent
Then it will get brighter and more vivid... By the time it reached 5" you will have an idea of how large (adult will look like)