In magical worlds things are ‘good’ or ‘evil’ because they feed and grow off of specific kinds of magical, spiritual energies. This is made fairly clear in Lord of the Rings and made brutally obvious in Star Wars. Terry Pratchett also explains in his book ‘Small Gods’ how gods live off of worshiper vibes (hopes, dreams, belief and will) - and become what energy they are given. You are what you eat after all!
The D&D pantheon, by design or not, follows this model really well. Demons don’t successfully attack Heaven - they starve out there (as would angels in the Abyss). Powerful fae like hags do not seek to become pretty even though they could easily do so - they feed off of dread, misery and despair. That’s their bag.
Dragons are magical and almost (but not quite) immortal. They may feed off of beliefs too - hence they are so obviously non-aerodynamic, lack camouflage, collect (useless) gold and use stupid tactics like confronting an entire army head on. They dress to impress. Also remember, D&D places the highest ranking colour & metallic dragons right in the Outer Planes (Celestia & Hell) - dragons are obviously quasi-planar creatures.
You can do whatever you like with your red dragons. That said, in any decent campaign: if one succeeded in becoming (genuinely) lawful good, it may look to the SHAPE of a red (perhaps?) - but have a colour-sheen of a deep red gold. A green dragon turning good would look obviously like a tarnished bronze, brass or copper. A black turning good would look weirdly like tarnished silver. Also, suck dragons would be horribly sick. They are really screwing up their energy source and more than half their food supply. Dragons are not the most powerful creatures that ever lived based thanks to a diet of horsemeat. That’s a griffon, not a dragon.
To be more clear: in 1st edition D&D a change of alignment meant the loss of levels, serious trauma… and your god would hunt you down. This has been lost in 5e for simplicity sake (yay?), but a magical creature changing alignments has far worse penalties. Let me give some examples to drive this point home.
A good hag would be obviously really pretty without trying, even if she tried to appear as elderly. The combined force of the entire haggate emporium would set themselves to have her torture-slaughtered over a decade or more.
An evil unicorn or pegasus would, Rules As Written, become a Nightmare if it didn’t die outright.
A good demon would be unable to stand their Abyssal home (‘the very wind would itch and chafe on their skin)’) - feeling more at home in the fae’s Faywilds. In fact, according to mythology, all demons are fallen angels and all fae are fallen angels that fell half way (to our world - hence all the Scottish and Irish lore around having so many fairies about).
If a Jedi were to fall, even if he were the Chosen One himself, why… he would slaughter off a bunch of younglings in minutes and then try to hunt down his most beloved friend - possibly slaughter his own wife (at great range) by accident.
If an angel were to fall in Lord of the Rings it would suddenly twist elves into orcs and make powerful magical items to control everyone directly.
See the pattern here?
In 5e people piss all over the alignment system and say it doesn’t matter. It does. It changes things in a way that transforms your entire story. Please pay attention to it.
