Verify ColorBuilder throws on null and empty

tests/ImageSharp.Tests/PixelFormats/ColorBuilderTests.cs

@@ -0,0 +1,31 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using SixLabors.ImageSharp.PixelFormats;
+using Xunit;
+
+namespace SixLabors.ImageSharp.Tests.Colors
+{
+    public class ColorBuilderTests
+    {
+        [Fact]
+        public void ParseHexLeadingPoundIsOptional()
+        {
+            Assert.Equal(new Rgb24(0, 128, 128), ColorBuilder<Rgb24>.FromHex("#008080"));
+            Assert.Equal(new Rgb24(0, 128, 128), ColorBuilder<Rgb24>.FromHex("008080"));
+        }
+
+        [Fact]
+        public void ParseHexThrowsOnEmpty()
+        {
+            Assert.Throws<ArgumentException>(() => ColorBuilder<Rgb24>.FromHex(""));
+        }
+
+        [Fact]
+        public void ParseHexThrowsOnNull()
+        {
+            Assert.Throws<ArgumentNullException>(() => ColorBuilder<Rgb24>.FromHex(null));
+        }
+    }
+}