Add default background and foreground color as possible colors

.editorconfig

@@ -1,3 +1,8 @@
+[*]
+tab_width = 4
+indent_size = 4
+indent_style = space
+
 [*.cs]
 dotnet_diagnostic.RCS1196.severity = none
 csharp_style_namespace_declarations = file_scoped:error

src/ConsoleApp/FileTime.ConsoleUI/Startup.cs

@@ -60,7 +60,7 @@ public static class Startup
                 DefaultForegroundColor: null,
                 DefaultForegroundAccentColor: colorProvider.RedForeground,
                 DefaultBackgroundColor: null,
-                ElementColor: colorProvider.GrayForeground,
+                ElementColor: colorProvider.DefaultForeground,
                 ContainerColor: colorProvider.BlueForeground,
                 MarkedItemForegroundColor: colorProvider.YellowForeground,
                 MarkedItemBackgroundColor: null,

src/Library/TerminalUI/Color/AnsiColorProvider.cs

@@ -25,6 +25,7 @@ public class AnsiColorProvider : ColorProviderBase
 
     public override IColor FromRgb(Rgb rgb, ColorType type) => new ColorRgb(rgb.R, rgb.G, rgb.B, type);
 
+    public override IColor DefaultForeground { get; } = Color256.DefaultForeground;
     public override IColor BlackForeground { get; } = new Color256(0, ColorType.Foreground);
     public override IColor BlueForeground { get; } = new Color256(12, ColorType.Foreground);
     public override IColor CyanForeground { get; } = new Color256(14, ColorType.Foreground);
@@ -42,6 +43,7 @@ public class AnsiColorProvider : ColorProviderBase
     public override IColor WhiteForeground { get; } = new Color256(15, ColorType.Foreground);
     public override IColor YellowForeground { get; } = new Color256(11, ColorType.Foreground);
 
+    public override IColor DefaultBackground { get; } = Color256.DefaultBackground;
     public override IColor BlackBackground { get; } = new Color256(0, ColorType.Background);
     public override IColor BlueBackground { get; } = new Color256(12, ColorType.Background);
     public override IColor CyanBackground { get; } = new Color256(14, ColorType.Background);

src/Library/TerminalUI/Color/Color256.cs

@@ -2,17 +2,42 @@
 
 namespace TerminalUI.Color;
 
-public readonly record struct Color256(byte Color, ColorType Type) : IColor
+public readonly record struct Color256(byte? Color, ColorType Type) : IColor
 {
     public string ToConsoleColor()
-        => Type switch
+    {
+        if (Color is null)
+        {
+            return string.Empty;
+        }
+        return Type switch
         {
             ColorType.Foreground => $"\x1b[38;5;{Color}m",
             ColorType.Background => $"\x1b[48;5;{Color}m",
-            _ => throw new InvalidEnumArgumentException(nameof(Type), (int) Type, typeof(ColorType))
+            _ => throw new InvalidEnumArgumentException(nameof(Type), (int)Type, typeof(ColorType))
+        };
+    }
+
+    public static Color256 DefaultForeground { get; } = new(null, ColorType.Foreground);
+    public static Color256 DefaultBackground { get; } = new(null, ColorType.Background);
+    private static Color256 DefaultForegroundAsBackground { get; } = new(7, ColorType.Background);
+    private static Color256 DefaultBackgroundAsForeground { get; } = new(0, ColorType.Foreground);
+
+    private static Dictionary<Color256, Color256> _defaultColorMapping = new()
+    {
+        { DefaultForeground, DefaultForegroundAsBackground },
+        { DefaultBackground, DefaultBackgroundAsForeground },
+        { DefaultForegroundAsBackground, DefaultForeground },
+        { DefaultBackgroundAsForeground, DefaultBackground },
     };
 
-    public IColor AsForeground() => this with {Type = ColorType.Foreground};
+    public IColor AsForeground()
+        => _defaultColorMapping.TryGetValue(this, out var color)
+            ? color
+            : this with { Type = ColorType.Foreground };
 
-    public IColor AsBackground() => this with {Type = ColorType.Background};
+    public IColor AsBackground()
+        => _defaultColorMapping.TryGetValue(this, out var color)
+            ? color
+            : this with { Type = ColorType.Background };
 }

src/Library/TerminalUI/Color/ColorProviderBase.cs

@@ -82,6 +82,7 @@ public abstract class ColorProviderBase : IColorProvider
             _ => null
         };
 
+    public abstract IColor DefaultForeground { get; }
     public abstract IColor BlackForeground { get; }
     public abstract IColor BlueForeground { get; }
     public abstract IColor CyanForeground { get; }
@@ -99,6 +100,7 @@ public abstract class ColorProviderBase : IColorProvider
     public abstract IColor WhiteForeground { get; }
     public abstract IColor YellowForeground { get; }
 
+    public abstract IColor DefaultBackground { get; }
     public abstract IColor BlackBackground { get; }
     public abstract IColor BlueBackground { get; }
     public abstract IColor CyanBackground { get; }

src/Library/TerminalUI/Color/ConsoleColorProvider.cs

@@ -73,6 +73,7 @@ public class ConsoleColorProvider : ColorProviderBase
     protected IColor ApproximateColor(Rgb rgb, IEnumerable<(Rgb, IColor)> colors)
         => colors.MinBy(c => c.Item1 - rgb).Item2;
 
+    public override IColor DefaultForeground => _blackForeground;
     public override IColor BlackForeground => _blackForeground;
     public override IColor BlueForeground => _blueForeground;
     public override IColor CyanForeground => _cyanForeground;
@@ -90,6 +91,7 @@ public class ConsoleColorProvider : ColorProviderBase
     public override IColor WhiteForeground => _whiteForeground;
     public override IColor YellowForeground => _yellowForeground;
 
+    public override IColor DefaultBackground => _blackBackground;
     public override IColor BlackBackground => _blackBackground;
     public override IColor BlueBackground => _blueBackground;
     public override IColor CyanBackground => _cyanBackground;

src/Library/TerminalUI/Color/IColorProvider.cs

@@ -5,6 +5,7 @@ public interface IColorProvider
     IColor Parse(string color, ColorType type);
     IColor FromRgb(Rgb rgb, ColorType type);
 
+    IColor DefaultForeground { get; }
     IColor BlackForeground { get; }
     IColor BlueForeground { get; }
     IColor CyanForeground { get; }
@@ -22,7 +23,7 @@ public interface IColorProvider
     IColor WhiteForeground { get; }
     IColor YellowForeground { get; }
 
-
+    IColor DefaultBackground { get; }
     IColor BlackBackground { get; }
     IColor BlueBackground { get; }
     IColor CyanBackground { get; }