Well keeping in mind I'm not TELLING anyone how to play their character (but am always up for offering advice and guidance if asked..).
I would think it would perfectly fine to RP a dragon becoming stronger in XYZ power associated with a breath upon ascending, and that forcing his colors to stick. I can see that an acceptable stretch of "lore" so to speak wihtout blowing it to bits
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Now the whole "dragon changes colors" for whatever reason, shifting for whatever reason is a bit more of a "blowing apart" of lore to me. Which is probably why you're being met with confusion from other RPers. I can see the logic of "ok I'm experimenting with available breath abilities to pick which one I shall concentrate on for my acienthood". But the shifting-according-to-what-power-I'm-Using (if that is what you're doing, I wasn't sure from yhour description) to me is a bit harder to allow because it is lorebreaking in more than one way, and it creates issues with "if your dragon changres why doesn't every other dragon?" Which leads to your dragon being an entirely different dragon species or something somehow...
Which to *me* goes too far. The one-time-shift/change say of the color upon ancient ascension due to your choosing to stick to one breath-power is far easier "lore jump" and simply explained by any dragon you may come into conctact with (and after the fact it really isn't a major part of RP anymore as you're just the one color combo or whatever). A constantly shifting color dragon is something much harder for say a "native Istarian" to wrap their brain around. If I am looking at you and you shift from red to black to blue, then my character has just seen something that has never existed and her brain explodes
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Also keep in mind that 1-dragon re-customization could still be a year or more away 2-it could be a one-time-use thing or a very limited use-thing 3-could be expensive RL money for each turn. So trying to come up with a continuing-RP-color-changing dragon to use this willy nilly may not actually be what's coming
. I don't see that as the main "basis" for what you want to do, but just pointing that out for your musings on how you want to envision your character
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