Edit:
I may have misinterpreted that you wanted the visual color to change, lol. If you were just talking about the color mechanic, then the Color Reroll bar is actually the easier one to get. Though, the Variant C evobar will make it easier to get a white Noo-Long specifically, since that's the most common color for that variant. And again, let me know if you need any help finding items :)

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Hey there @Wind -- So, the Color Reroll bar doesn't change the visual color of a pet (I know, confusing), it only changes the color mechanic which determines what decorations and eggs your pet can find during hoards.

So I would definitely go for the Variant C evobar, since that one will actually change the visual color of your Noo-Long to white. In addition it will change the color mechanic too, since Albino Noo-Long has a different color set!

Good luck making the bar, let me know if you're looking for any items for it and I'll be happy to help :)

@Wind

(Sorry for the late reply) White dragon scales mostly come from hoarding -- I think you can find a few while adventuring, but not enough to make the bar. Other than that, you're correct about everything else!