Add an RGB color parser (#165)

src/WpfMath.Tests/TestResults/ParserTests.colorModels.(color [RGB] (128, 128, 128) x).approved.txt

@@ -0,0 +1,39 @@
+{
+  "TextStyle": null,
+  "RootAtom": {
+    "RowAtom": {
+      "PreviousAtom": null,
+      "Elements": [
+        {
+          "Character": "x",
+          "TextStyle": null,
+          "IsDefaultTextStyle": true,
+          "IsTextSymbol": false,
+          "Type": "Ordinary",
+          "Source": {
+            "Start": 29,
+            "End": 30,
+            "Length": 1,
+            "Source": "\\color [RGB] {128, 128, 128} x"
+          }
+        }
+      ],
+      "Type": "Ordinary",
+      "Source": {
+        "Start": 1,
+        "End": 30,
+        "Length": 29,
+        "Source": "\\color [RGB] {128, 128, 128} x"
+      }
+    },
+    "Background": null,
+    "Foreground": "#FF808080",
+    "Type": "Ordinary",
+    "Source": {
+      "Start": 1,
+      "End": 30,
+      "Length": 29,
+      "Source": "\\color [RGB] {128, 128, 128} x"
+    }
+  }
+}

src/WpfMath.Tests/TestResults/ParserTests.colorModels.(colorbox [RGB] (128, 128, 128) x).approved.txt

@@ -0,0 +1,39 @@
+{
+  "TextStyle": null,
+  "RootAtom": {
+    "RowAtom": {
+      "PreviousAtom": null,
+      "Elements": [
+        {
+          "Character": "x",
+          "TextStyle": null,
+          "IsDefaultTextStyle": true,
+          "IsTextSymbol": false,
+          "Type": "Ordinary",
+          "Source": {
+            "Start": 32,
+            "End": 33,
+            "Length": 1,
+            "Source": "\\colorbox [RGB] {128, 128, 128} x"
+          }
+        }
+      ],
+      "Type": "Ordinary",
+      "Source": {
+        "Start": 1,
+        "End": 33,
+        "Length": 32,
+        "Source": "\\colorbox [RGB] {128, 128, 128} x"
+      }
+    },
+    "Background": "#FF808080",
+    "Foreground": null,
+    "Type": "Ordinary",
+    "Source": {
+      "Start": 1,
+      "End": 33,
+      "Length": 32,
+      "Source": "\\colorbox [RGB] {128, 128, 128} x"
+    }
+  }
+}

src/WpfMath/Colors/RgbColorParser.cs

@@ -0,0 +1,27 @@
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using System.Windows.Media;
+
+namespace WpfMath.Colors
+{
+    public class RgbColorParser : IColorParser
+    {
+        public Color? Parse(IEnumerable<string> components)
+        {
+            var rgbStrings = components.ToList();
+            if (rgbStrings.Count != 3)
+                return null;
+
+            var rgb = rgbStrings.Select(x =>
+                {
+                    var success = byte.TryParse(x, NumberStyles.None, CultureInfo.InvariantCulture, out var val);
+                    return success ? (byte?) val : null;
+                }).ToArray();
+            var r = rgb[0];
+            var g = rgb[1];
+            var b = rgb[2];
+            return r == null || g == null || b == null ? (Color?) null : Color.FromRgb(r.Value, g.Value, b.Value);
+        }
+    }
+}

src/WpfMath/Colors/StandardColorParsers.cs

@@ -7,7 +7,8 @@ internal static class StandardColorParsers
         public static IReadOnlyDictionary<string, IColorParser> Dictionary = new Dictionary<string, IColorParser>
         {
             ["gray"] = new GrayscaleColorParser(),
-            ["HTML"] = new HtmlColorParser()
+            ["HTML"] = new HtmlColorParser(),
+            ["RGB"] = new RgbColorParser()
         };
     }
 }