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src/libraries/System.Drawing.Primitives/src/System/Drawing/KnownColorTable.cs

@@ -427,6 +427,11 @@ internal static class KnownColorTable
  KnownColorKindWeb, // RebeccaPurple
  ];
 
+ // These values were based on manual investigation of dark mode themes in the
+ // Win32 Common Controls and WinUI. There aren't direct mappings published by
+ // Windows, these may change slightly when this feature is finalized to make
+ // sure we have the best experience in hybrid dark mode scenarios (mixing
+ // WPF, WinForms, and WinUI).
  private static ReadOnlySpan<uint> AlternateSystemColors =>
  [
  0, // To align with KnownColor.ActiveBorder = 1
@@ -494,18 +499,24 @@ public static uint KnownColorToArgb(KnownColor color)
  : ColorValueTable[(int)color];
  }
 
+ private static uint GetAlternateSystemColorArgb(KnownColor color)
+ {
+ // Shift the original (split) index to fit the alternate color map.
+ int index = color <= KnownColor.WindowText
+ ? (int)color
+ : (int)color - (int)KnownColor.ButtonFace + (int)KnownColor.WindowText + 1;
+
+ return AlternateSystemColors[index];
+ }
+
 #if FEATURE_WINDOWS_SYSTEM_COLORS
  public static uint GetSystemColorArgb(KnownColor color)
  {
  Debug.Assert(Color.IsKnownColorSystem(color));
 
- if (!SystemColors.s_useAlternativeColorSet || HighContrastEnabled())
- {
- return ColorTranslator.COLORREFToARGB(Interop.User32.GetSysColor((byte)ColorValueTable[(int)color]));
- }
-
- int index = color <= KnownColor.WindowText ? (int)color : (int)color - (int)KnownColor.ButtonFace + (int)KnownColor.WindowText + 1;
- return AlternateSystemColors[index];
+ return !SystemColors.s_useAlternativeColorSet || HighContrastEnabled()
+ ? ColorTranslator.COLORREFToARGB(Interop.User32.GetSysColor((byte)ColorValueTable[(int)color]))
+ : GetAlternateSystemColorArgb(color);
  }
 
  private static unsafe bool HighContrastEnabled()
@@ -529,13 +540,9 @@ public static uint GetSystemColorArgb(KnownColor color)
  {
  Debug.Assert(Color.IsKnownColorSystem(color));
 
- if (!SystemColors.s_useAlternativeColorSet)
- {
- return ColorValueTable[(int)color];
- }
-
- int index = color <= KnownColor.WindowText ? (int)color : (int)color - (int)KnownColor.ButtonFace + (int)KnownColor.WindowText + 1;
- return AlternateSystemColors[index];
+ return (!SystemColors.s_useAlternativeColorSet)
+ ? ColorValueTable[(int)color]
+ : GetAlternateSystemColorArgb(color);
  }
 #endif
  }

src/libraries/System.Drawing.Primitives/src/System/Drawing/SystemColors.cs

@@ -60,6 +60,11 @@ public static class SystemColors
  /// </summary>
  /// <remarks>
  /// <para>
+ /// <see cref="KnownColor"/> <see cref="Color"/> values are always looked up every
+ /// time you use them and do not retain any other context. As such, existing
+ /// <see cref="Color"/> values will change when this property is set.
+ /// </para>
+ /// <para>
  /// On Windows, system <see cref="KnownColor"/> values will always return the current
  /// Windows color when the OS has a high contrast theme enabled.
  /// </para>