Fix the partial struct issue in #298

Former-commit-id: 1115e78624113394b0f522c69ceedf616bd525ec
Former-commit-id: 68d0059211d610e1c589899b9d6f0c15b97aa3b7
Former-commit-id: 0912a58e2c372467a6a89a76d70db81a68f45202

src/ImageProcessor/Colors/ColorSpacialTransforms.cs

@@ -0,0 +1,31 @@
+// <copyright file="ColorSpacialTransforms.cs" company="James Jackson-South">
+// Copyright (c) James Jackson-South and contributors.
+// Licensed under the Apache License, Version 2.0.
+// </copyright>
+
+namespace ImageProcessor
+{
+    using System.Numerics;
+
+    public partial struct Color
+    {
+        public static Color Multiply(Color source, Color destination)
+        {
+            if (destination == Color.Black)
+            {
+                return Color.Black;
+            }
+            if (destination == Color.White)
+            {
+                return source;
+            }
+            return
+                new Color(
+                    new Vector4(
+                        source.backingVector.X * destination.backingVector.X,
+                        source.backingVector.Y * destination.backingVector.Y,
+                        source.backingVector.Z * destination.backingVector.Z,
+                        source.backingVector.W * destination.backingVector.W));
+        }
+    }
+}

tests/ImageProcessor.Tests/Colors/ColorSpacialTransformTests.cs

@@ -0,0 +1,53 @@
+namespace ImageProcessor.Tests
+{
+    using Xunit;
+
+    public class ColorSpacialTransformTests
+    {
+        public class MultiplyTests
+        {
+            [Fact]
+            public void MultiplyBlendConvertsRedBackdropAndGreenOverlayToBlack()
+            {
+                var backdrop = Color.Red;
+                var overlay = Color.Green;
+
+                var result = Color.Multiply(backdrop, overlay);
+
+                Assert.Equal(Color.Black, result);
+            }
+            [Fact]
+            public void MultiplyBlendConvertsBlueBackdropAndWhiteOverlayToBlue()
+            {
+                var backdrop = Color.Blue;
+                var overlay = Color.White;
+
+                var result = Color.Multiply(backdrop, overlay);
+
+                Assert.Equal(Color.Blue, result);
+            }
+            [Fact]
+            public void MultiplyBlendConvertsBlueBackdropAndBlackOverlayToBlack()
+            {
+                var backdrop = Color.Blue;
+                var overlay = Color.Black;
+
+                var result = Color.Multiply(backdrop, overlay);
+
+                Assert.Equal(Color.Black, result);
+            }
+            [Fact]
+            public void MultiplyBlendConvertsBlueBackdropAndGrayOverlayToBlueBlack()
+            {
+                var backdrop = Color.Blue;
+                var overlay = Color.Gray;
+
+                var result = Color.Multiply(backdrop, overlay);
+
+                var expected = new Color(0, 0, 0.5f, 1);
+
+                Assert.True(expected.AlmostEquals(result,.01f));
+            }
+        }
+    }
+}