use ANSI escape codes to control colours of parameters we print

src/D2L.Bmx/ConsoleWriter.cs

@@ -4,14 +4,21 @@ internal interface IConsoleWriter {
 	void WriteParameter( string description, string value, ParameterSource source );
 }
 
+// We use ANSI escape codes to control colours, because .NET's `Console.ForegroundColor` only targets stdout,
+// if stdout is redirected (e.g. typical use case for `bmx print`), we won't get any coloured text on stderr.
+// https://github.com/dotnet/runtime/issues/83146
 internal class ConsoleWriter : IConsoleWriter {
+	// .NET runtime subscribes to the informal standard from https://no-color.org/. We should too.
+	// https://github.com/dotnet/runtime/blob/v9.0.0-preview.6.24327.7/src/libraries/Common/src/System/Console/ConsoleUtils.cs#L32-L34
+	private static readonly bool _noColor = Environment.GetEnvironmentVariable( "NO_COLOR" ) == "1";
+
 	void IConsoleWriter.WriteParameter( string description, string value, ParameterSource source ) {
-		Console.ResetColor();
-		Console.Error.Write( $"{description}: " );
-		Console.ForegroundColor = ConsoleColor.Cyan;
-		Console.Error.Write( value );
-		Console.ForegroundColor = ConsoleColor.DarkGray;
-		Console.Error.WriteLine( $" (from {source})" );
-		Console.ResetColor();
+		if( _noColor || !VirtualTerminal.TryEnableOnStderr() ) {
+			Console.Error.WriteLine( $"{description}: {value} (from {source})" );
+		}
+		// description: default
+		// value: bright cyan
+		// source: grey / bright black
+		Console.Error.WriteLine( $"\x1b[0m{description}: \x1b[96m{value} \x1b[90m(from {source})\x1b[0m" );
 	}
 }

src/D2L.Bmx/VirtualTerminal.cs

@@ -0,0 +1,54 @@
+using System.Runtime.InteropServices;
+
+namespace D2L.Bmx;
+
+// The code here exists because legacy Windows console doesn't support ANSI escape codes by default.
+// We can consider deleting everything here once Windows 10 is out of support and
+// Windows Terminal becomes the default console on all supported Windows versions.
+internal static partial class VirtualTerminal {
+	private const string Kernel32 = "kernel32.dll";
+	private const int STD_ERROR_HANDLE = -12;
+	private const uint ENABLE_VIRTUAL_TERMINAL_PROCESSING = 0x0004;
+
+	private static bool _enablingAttempted = false;
+	private static bool _enabled = false;
+
+	// https://learn.microsoft.com/en-us/windows/console/classic-vs-vt
+	public static bool TryEnableOnStderr() {
+		if( _enablingAttempted ) {
+			return _enabled;
+		}
+		_enablingAttempted = true;
+
+		if( !RuntimeInformation.IsOSPlatform( OSPlatform.Windows ) ) {
+			// POSIX systems all support ANSI escape codes
+			_enabled = true;
+			return true;
+		}
+		nint stderrHandle = GetStdHandle( STD_ERROR_HANDLE );
+		if( !GetConsoleMode( stderrHandle, out int mode ) ) {
+			_enabled = false;
+			return false;
+		}
+		if( ( mode & ENABLE_VIRTUAL_TERMINAL_PROCESSING ) == ENABLE_VIRTUAL_TERMINAL_PROCESSING ) {
+			_enabled = true;
+			return true;
+		}
+		_enabled = SetConsoleMode( stderrHandle, (int)( mode | ENABLE_VIRTUAL_TERMINAL_PROCESSING ) );
+		return _enabled;
+	}
+
+	// https://learn.microsoft.com/en-us/windows/console/getstdhandle
+	[LibraryImport( Kernel32 )]
+	private static partial IntPtr GetStdHandle( int nStdHandle );
+
+	// https://learn.microsoft.com/en-us/windows/console/getconsolemode
+	[LibraryImport( Kernel32 )]
+	[return: MarshalAs( UnmanagedType.Bool )]
+	private static partial bool GetConsoleMode( IntPtr handle, out int mode );
+
+	// https://learn.microsoft.com/en-us/windows/console/setconsolemode
+	[LibraryImport( Kernel32 )]
+	[return: MarshalAs( UnmanagedType.Bool )]
+	private static partial bool SetConsoleMode( IntPtr handle, int mode );
+}