Introduce new ColorScheme roles for Material 3

New color roles in ColorScheme include seven tone-based surfaces and containers, and twelve accent colors for primary, secondary, and tertiary groups. This update deprecates three existing color roles: background, onBackground, and surfaceVariant. The ColorScheme constructed by the updated ColorScheme.fromSeed method now generates different values compared to the previous version, adapting to the Material Design 3 guidelines.

The tone-based surface colors include:

• surfaceBright
• surfaceDim
• surfaceContainer
• surfaceContainerLow
• surfaceContainerLowest
• surfaceContainerHigh
• surfaceContainerHighest

These changes help eliminate the use of widgets' surfaceTintColor, and replaces the old opacity-based model that applied a tinted overlay on top of surfaces based on their elevation.

The default surfaceTintColor for all widgets is now null and their default background color is now based on the new tone-based surface colors.

ColorScheme.fromSeed has also been updated to use the latest algorithm from the Material color utilities package. This change prevents the constructed ColorScheme from being too bright, even if the source color looks bright and had a high chroma (contained little black, white, and shades of grey).

The differences caused by the updated ColorScheme.fromSeed and the new color roles should be small and acceptable. However, when providing a brighter seed color to ColorScheme.fromSeed, it might construct a relatively darker version of ColorScheme. To force the output to still be bright, set dynamicSchemeVariant: DynamicSchemeVariant.fidelity in ColorScheme.fromSeed. For example:

Code before migration:

ColorScheme.fromSeed(
    seedColor: Color(0xFF0000FF), // Bright blue
)

Code after migration:

ColorScheme.fromSeed(
    seedColor: Color(0xFF0000FF), // Bright blue
    dynamicSchemeVariant: DynamicSchemeVariant.fidelity,
)

Material Design 3 removes 3 colors.

To configure the appearance of the material components, background should be replaced with surface, onBackground should be replaced with onSurface, and surfaceVariant should be migrated to surfaceContainerHighest.

Code before migration:

final ColorScheme colorScheme = ColorScheme();
MaterialApp(
  theme: ThemeData(
    //...
    colorScheme: colorScheme.copyWith(
      background: myColor1,
      onBackground: myColor2,
      surfaceVariant: myColor3,
    ),
  ),
  //...
)

Code after migration:

final ColorScheme colorScheme = ColorScheme();
MaterialApp(
  theme: ThemeData(
    //...
    colorScheme: colorScheme.copyWith(
      surface: myColor1,
      onSurface: myColor2,
      surfaceContainerHighest: myColor3,
    ),
  ),
  //...
)

Custom components that used to look up ColorScheme.background, ColorScheme.onBackground, and ColorScheme.surfaceVariant can look up ColorScheme.surface, ColorScheme.onSurface and ColorScheme.surfaceContainerHighest instead.

Code before migration:

Color myColor1 = Theme.of(context).colorScheme.background;
Color myColor2 = Theme.of(context).colorScheme.onBackground;
Color myColor3 = Theme.of(context).colorScheme.surfaceVariant;

Code after migration:

Color myColor1 = Theme.of(context).colorScheme.surface;
Color myColor2 = Theme.of(context).colorScheme.onSurface;
Color myColor3 = Theme.of(context).colorScheme.surfaceContainerHighest;

Landed in version: 3.21.0-4.0.pre
In stable release: 3.22.0

Relevant issues:

• Support tone-based surface and surface container ColorScheme roles
• Support fidelity variant for ColorScheme.fromSeed

Relevant PRs:

• Introduce tone-based surfaces and accent color add-ons - Part 1
• Introduce tone-based surfaces and accent color add-ons - Part 2
• Enhance ColorScheme.fromSeed with a new variant parameter

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