This package helps to determine if a color is dark tone or light tone. It's useful to use a contrast color (as you can see on demo image using on an app) with other. It's based on HSP(Highly Sensitive Poo) color model, you can read more about it here.
npm install --save is-dark-color-hsp
import isDarkColorHsp from 'is-dark-color'
const whiteIsDark = isDarkColorHsp.isDark('#ffffff'); // false
const blackIsDark = isDarkColorHsp.isDark('rgb(0, 0, 0)'); // true
const otherColor = isDarkColorHsp.isLight('#ff9900'); // true
const otherColor = isDarkColorHsp.isDark('hsl(142, 42%, 42%)'); // true
const nonColor = isDarkColorHsp.isDark({}); // null
See this package for Flutter here.