To add the package css_colors to an app:
Depend on it
Open the pubspec.yaml file located inside the app folder, and add css_colors: ^1.0.0 under dependencies.
Install it
From the terminal: Run flutter pub get.
OR
From VS Code: Click Get Packages located in right side of the action ribbon at the top of pubspec.yaml indicated by the Download icon.
From Android Studio/IntelliJ: Click Pub get in the action ribbon at the top of pubspec.yaml.
Import it
Add a corresponding import statement in the Dart code.
Stop and restart the app, if necessary
If the package brings platform-specific code (Kotlin/Java for Android, Swift/Objective-C for iOS), that code must be built into your app. Hot reload and hot restart only update the Dart code, so a full restart of the app might be required to avoid errors like MissingPluginException when using the package.