and it was the influence of the Rider Bond spell, and possibly other enchantments by the elves, that gave the Rider dragons such unnatural and showy colours.
I think so too. I could see the elves giving them all kinds of bright colours to be able to show them off, and when the Riders were fully established, they diversified even further. And because they effectively ruled the country, there wouldn't be negative effects.
This would also explain why Saphira and Glaedr are so sparkly. That would be a cosmetic choice the Rider dragons made to be more impressive, while the wild dragons kept dull scales.