It would be easier to just buy a whole replacement backplate.

https://www.amazon.com/Deal4GO-Official-Backplate-Replacement-Nintendo/dp/B07ZRWBW36

Color matching plastic paint would be difficult, and potentially more expensive. Some of those little paint pots can easily run $15 -$20, and may not turn out to be an exact match. Even if you got two or shades to mix, then you're just looking at more money.