diff --git a/Semantics.Color/Color.Conversions.cs b/Semantics.Color/Color.Conversions.cs
index c13a1e6..2bce541 100644
--- a/Semantics.Color/Color.Conversions.cs
+++ b/Semantics.Color/Color.Conversions.cs
@@ -45,7 +45,7 @@ public Vector3 ToSrgbVector3()
 	/// <param name="hex">The hex color string.</param>
 	/// <returns>The linear-RGB color.</returns>
 	/// <exception cref="ArgumentNullException">Thrown when <paramref name="hex"/> is null.</exception>
-	/// <exception cref="ArgumentException">Thrown when <paramref name="hex"/> is not a recognised hex length.</exception>
+	/// <exception cref="ArgumentException">Thrown when <paramref name="hex"/> is not a recognised hex length, or contains a character that is not a hex digit.</exception>
 	public static Color FromHex(string hex)
 	{
 		Ensure.NotNull(hex);
@@ -62,6 +62,16 @@ public static Color FromHex(string hex)
 			throw new ArgumentException("Hex color must be #RGB, #RRGGBB, or #RRGGBBAA.", nameof(hex));
 		}
 
+		// Convert.ToByte(s, 16) accepts a leading '+' and throws FormatException for other bad
+		// characters, so every character is checked here and only hex digits reach it.
+		foreach (char c in h)
+		{
+			if (!IsHexDigit(c))
+			{
+				throw new ArgumentException($"Hex color contains '{c}', which is not a hex digit.", nameof(hex));
+			}
+		}
+
 		byte r = ParseByte(h, 0);
 		byte g = ParseByte(h, 2);
 		byte b = ParseByte(h, 4);
@@ -99,6 +109,9 @@ public static Color FromBytes(byte r, byte g, byte b, byte a = 255) =>
 	private static byte ParseByte(string hex, int index) =>
 		Convert.ToByte(hex.Substring(index, 2), 16);
 
+	private static bool IsHexDigit(char c) =>
+		c is (>= '0' and <= '9') or (>= 'A' and <= 'F') or (>= 'a' and <= 'f');
+
 	/// <summary>Converts this linear color to <see cref="Oklab"/>.</summary>
 	/// <returns>The Oklab equivalent.</returns>
 	public Oklab ToOklab() => Oklab.FromColor(this);
diff --git a/Semantics.Test/Colors/HexConversionTests.cs b/Semantics.Test/Colors/HexConversionTests.cs
index e92d49c..e13682e 100644
--- a/Semantics.Test/Colors/HexConversionTests.cs
+++ b/Semantics.Test/Colors/HexConversionTests.cs
@@ -58,4 +58,27 @@ public void ToHex_EmitsAlphaWhenNotOpaque()
 	[TestMethod]
 	public void FromHex_InvalidLength_Throws() =>
 		Assert.ThrowsExactly<ArgumentException>(() => Color.FromHex("#12345"));
+
+	[TestMethod]
+	[DataRow("#+F+F+F")]
+	[DataRow("+F+F+F")]
+	[DataRow("#00+F00")]
+	[DataRow("#GGGGGG")]
+	[DataRow("#0x0x0x")]
+	[DataRow("#-1-1-1")]
+	[DataRow("#12345G")]
+	[DataRow("#FFFFFF+F")]
+	[DataRow("#F F")]
+	[DataRow("#G00")]
+	public void FromHex_NonHexCharacter_ThrowsArgumentException(string hex) =>
+		Assert.ThrowsExactly<ArgumentException>(() => Color.FromHex(hex));
+
+	[TestMethod]
+	[DataRow("#abc", "#AABBCC")]
+	[DataRow("#ABC", "#AABBCC")]
+	[DataRow("#3a7bd5", "#3A7BD5")]
+	[DataRow("3A7BD5", "#3A7BD5")]
+	[DataRow("#3a7bd580", "#3A7BD580")]
+	public void FromHex_ValidDigitsInEitherCase_RoundTrip(string hex, string expected) =>
+		Assert.AreEqual(expected, Color.FromHex(hex).ToHex());
 }