diff --git a/Schema.Test/CodeGenerationRoundTripTests.cs b/Schema.Test/CodeGenerationRoundTripTests.cs
index d9a76c1..14e8ec7 100644
--- a/Schema.Test/CodeGenerationRoundTripTests.cs
+++ b/Schema.Test/CodeGenerationRoundTripTests.cs
@@ -230,6 +230,68 @@ public void TestAPromisingClassStartsAtItsDefaults()
 		Assert.AreEqual(2.5f, triple.GetProperty("X")!.GetValue(instance));
 	}
 
+	/// <summary>
+	/// A member of a semantic type, of a vector or of a colour starts at its declared default,
+	/// the value the C++ generator starts it at.
+	/// </summary>
+	/// <remarks>
+	/// Validation accepts a numeric default on all of these, and the attribute carrying it was
+	/// always written - but with no initialiser, a fresh instance started at zero. Reading the
+	/// attribute back cannot see that; only constructing an instance can.
+	/// </remarks>
+	/// <param name="travelsAsBytes">Whether the class is a struct, which needs a constructor
+	/// before its members may have initialisers.</param>
+	[TestMethod]
+	[DataRow(false)]
+	[DataRow(true)]
+	public void TestSemanticVectorAndColourMembersStartAtTheirDefaults(bool travelsAsBytes)
+	{
+		Schema original = new();
+		original.AddSemanticType("Kilograms".As<SemanticTypeName>())!.SetUnderlyingType(new Models.Types.Float());
+		original.AddSemanticType("Count".As<SemanticTypeName>())!.SetUnderlyingType(new Models.Types.Int());
+		original.AddSemanticType("Heavy".As<SemanticTypeName>())!.SetUnderlyingType(
+			new Models.Types.Semantic { SemanticTypeName = "Kilograms".As<SemanticTypeName>() });
+
+		SchemaClass body = original.AddClass("Body".As<ClassName>())!;
+		body.TravelsAsBytes = travelsAsBytes;
+		AddDefaulted(body, "Mass", new Models.Types.Semantic { SemanticTypeName = "Kilograms".As<SemanticTypeName>() }, 2.5);
+		AddDefaulted(body, "Offset", new Models.Types.Semantic { SemanticTypeName = "Kilograms".As<SemanticTypeName>() }, -1.5);
+		AddDefaulted(body, "Parts", new Models.Types.Semantic { SemanticTypeName = "Count".As<SemanticTypeName>() }, 3);
+		AddDefaulted(body, "Ballast", new Models.Types.Semantic { SemanticTypeName = "Heavy".As<SemanticTypeName>() }, 4);
+		AddDefaulted(body, "Scale", new Models.Types.Vector3(), 1);
+		AddDefaulted(body, "Cell", new Models.Types.Vector2 { ElementType = new Models.Types.Int() }, 2);
+		AddDefaulted(body, "Tint", new Models.Types.ColorRGBA(), 0.5);
+
+		SchemaGenerationResult result = SchemaGenerator.Generate(original, CodeGenerationTests.ConfigureGenerator(original));
+		Assert.IsTrue(result.IsSuccess, result.Message);
+
+		Assembly assembly = GeneratedSourceCompiler.Compile(result.Files);
+		Type type = assembly.GetType("Generated.Body", throwOnError: true)!;
+		object instance = Activator.CreateInstance(type)!;
+
+		Assert.AreEqual(2.5f, SemanticValue(instance, "Mass"));
+		Assert.AreEqual(-1.5f, SemanticValue(instance, "Offset"));
+		Assert.AreEqual(3, SemanticValue(instance, "Parts"));
+		Assert.AreEqual(4f, SemanticValue(instance, "Ballast"));
+		Assert.AreEqual(new System.Numerics.Vector3(1f), type.GetProperty("Scale")!.GetValue(instance));
+		Assert.AreEqual(Activator.CreateInstance(type.GetProperty("Cell")!.PropertyType, 2, 2), type.GetProperty("Cell")!.GetValue(instance));
+		Assert.AreEqual(new Runtime.ColorRgba(0.5f, 0.5f, 0.5f, 0.5f), type.GetProperty("Tint")!.GetValue(instance));
+
+		static void AddDefaulted(SchemaClass owner, string name, Models.Types.BaseType memberType, double value)
+		{
+			SchemaMember member = owner.AddMember(name.As<MemberName>())!;
+			member.SetType(memberType);
+			member.DefaultValue = new NumberDefault { Value = value };
+		}
+
+		// A semantic member's value, read through the Value property of the type it holds.
+		static object SemanticValue(object owner, string property)
+		{
+			object held = owner.GetType().GetProperty(property)!.GetValue(owner)!;
+			return held.GetType().GetProperty("Value")!.GetValue(held)!;
+		}
+	}
+
 	/// <summary>
 	/// A class that travels as bytes, holding one that does the same - which is what the promise
 	/// permits and what the C# side could not represent before.
diff --git a/Schema/Generation/CSharpCodeGenerator.cs b/Schema/Generation/CSharpCodeGenerator.cs
index 7edfe7b..1a862af 100644
--- a/Schema/Generation/CSharpCodeGenerator.cs
+++ b/Schema/Generation/CSharpCodeGenerator.cs
@@ -754,26 +754,72 @@ private static string ObjectInitialiserFor(Models.Types.Object objectType) =>
 	/// <summary>
 	/// Gets the initialiser for a member's default, or null when it has none that fits.
 	/// </summary>
+	private static string? DefaultInitialiserFor(SchemaMember member) =>
+		member.DefaultValue is { } value && DefaultLiteralFor(value, member.Type) is string literal
+			? $" = {literal};"
+			: null;
+
+	/// <summary>
+	/// Writes a default as an expression of the given type, or null when it has none that fits.
+	/// </summary>
 	/// <remarks>
+	/// Validation reads a default through a semantic type to its representation, and lets a single
+	/// number stand for every component of a vector, so both are written here the way the C++
+	/// generator writes them: a semantic type through its explicit conversion, and a vector with the
+	/// number in each component.
+	/// <para>
 	/// A default of a kind the member cannot hold is a validation error, and generation is refused
 	/// for a schema that has one - so the mismatched cases here are only reachable by calling this
 	/// generator directly on a schema that was never validated. They fall through to the type's
 	/// own initialiser rather than emitting source that does not compile.
+	/// </para>
 	/// </remarks>
-	private static string? DefaultInitialiserFor(SchemaMember member) => (member.DefaultValue, member.Type) switch
+	private static string? DefaultLiteralFor(MemberDefault value, BaseType type) => (value, type) switch
 	{
-		(NumberDefault number, Int) => $" = {(long)number.Value};",
-		(NumberDefault number, Long) => $" = {(long)number.Value}L;",
-		(NumberDefault number, Float) when !double.IsFinite(number.Value) => $" = float.{NonFinite(number.Value)};",
-		(NumberDefault number, Double) when !double.IsFinite(number.Value) => $" = double.{NonFinite(number.Value)};",
-		(NumberDefault number, Float) => $" = {number.Value.ToString("R", CultureInfo.InvariantCulture)}f;",
-		(NumberDefault number, Double) => $" = {Literal(number.Value)};",
-		(BooleanDefault boolean, Bool) => $" = {(boolean.Value ? "true" : "false")};",
-		(TextDefault text, Models.Types.String) => $" = {Quote(text.Value)};",
-		(TextDefault text, Models.Types.Enum enumType) => $" = {CSharpKeywords.Identifier(enumType.EnumName)}.{CSharpKeywords.Identifier(text.Value)};",
+		(NumberDefault number, Int) => $"{(long)number.Value}",
+		(NumberDefault number, Long) => $"{(long)number.Value}L",
+		(NumberDefault number, Float) when !double.IsFinite(number.Value) => $"float.{NonFinite(number.Value)}",
+		(NumberDefault number, Double) when !double.IsFinite(number.Value) => $"double.{NonFinite(number.Value)}",
+		(NumberDefault number, Float) => $"{number.Value.ToString("R", CultureInfo.InvariantCulture)}f",
+		(NumberDefault number, Double) => Literal(number.Value),
+		(BooleanDefault boolean, Bool) => boolean.Value ? "true" : "false",
+		(TextDefault text, Models.Types.String) => Quote(text.Value),
+		(TextDefault text, Models.Types.Enum enumType) => $"{CSharpKeywords.Identifier(enumType.EnumName)}.{CSharpKeywords.Identifier(text.Value)}",
+
+		// The colours before their vector bases, as in MapType: ColorRGB derives from Vector3.
+		(NumberDefault, ColorRGB or ColorRGBA) => ComponentsLiteral(value, type, new Float()),
+		(NumberDefault, Vector vector) => ComponentsLiteral(value, vector, vector.ElementType),
+
+		// A representation that is itself a semantic type is a refinement chain that never reached
+		// anything real, which validation reports; following it again would have no bottom.
+		(_, Semantic { Declaration: SchemaSemanticType declaration } semantic)
+			when declaration.Representation() is not Semantic
+			&& DefaultLiteralFor(value, declaration.Representation()) is string represented
+			=> $"({MapType(semantic)})({represented})",
+
 		_ => null,
 	};
 
+	/// <summary>
+	/// Writes a vector or colour default: one number, in every component.
+	/// </summary>
+	private static string? ComponentsLiteral(MemberDefault value, BaseType type, BaseType elementType)
+	{
+		if (DefaultLiteralFor(value, elementType) is not string component)
+		{
+			return null;
+		}
+
+		int count = type switch
+		{
+			Vector2 => 2,
+			Vector3 => 3,
+			_ => 4,
+		};
+
+		return $"new {MapType(type)}({string.Join(", ", Enumerable.Repeat(component, count))})";
+	}
+
 	/// <summary>
 	/// Names one of the two values a floating-point number can hold that has no literal.
 	/// </summary>