Add support for RGB colors

Closes #34

src/Spectre.Console.Tests/Unit/StyleTests.cs

@@ -153,6 +153,35 @@ namespace Spectre.Console.Tests.Unit
                 result.ShouldNotBeNull();
                 result.Message.ShouldBe(expected);
             }
+
+            [Theory]
+            [InlineData("rgb(255,0,0) on rgb(0,0,255)")]
+            public void Should_Parse_Rgb_Colors_Correctly(string style)
+            {
+                // Given, When
+                var result = Style.Parse(style);
+
+                // Then
+                result.Foreground.ShouldBe(Color.Red);
+                result.Background.ShouldBe(Color.Blue);
+            }
+
+            [Theory]
+            [InlineData("rgb()", "Invalid RGB color 'rgb()'.")]
+            [InlineData("rgb(", "Invalid RGB color 'rgb('.")]
+            [InlineData("rgb(255)", "Invalid RGB color 'rgb(255)'.")]
+            [InlineData("rgb(255,255)", "Invalid RGB color 'rgb(255,255)'.")]
+            [InlineData("rgb(255,255,255", "Invalid RGB color 'rgb(255,255,255'.")]
+            [InlineData("rgb(A,B,C)", "Invalid RGB color 'rgb(A,B,C)'. Input string was not in a correct format.")]
+            public void Should_Return_Error_If_Rgb_Color_Is_Invalid(string style, string expected)
+            {
+                // Given, When
+                var result = Record.Exception(() => Style.Parse(style));
+
+                // Then
+                result.ShouldNotBeNull();
+                result.Message.ShouldBe(expected);
+            }
         }
 
         public sealed class TheTryParseMethod

src/Spectre.Console/Internal/Text/StyleParser.cs

@@ -1,4 +1,6 @@
 using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Globalization;
 
 namespace Spectre.Console.Internal
 {
@@ -65,6 +67,14 @@ namespace Spectre.Console.Internal
                                 return null;
                             }
                         }
+                        else if (part.StartsWith("rgb", StringComparison.OrdinalIgnoreCase))
+                        {
+                            color = ParseRgbColor(part, out error);
+                            if (!string.IsNullOrWhiteSpace(error))
+                            {
+                                return null;
+                            }
+                        }
                         else
                         {
                             error = !foreground
@@ -102,6 +112,7 @@ namespace Spectre.Console.Internal
             return new Style(effectiveForeground, effectiveBackground, effectiveDecoration);
         }
 
+        [SuppressMessage("Design", "CA1031:Do not catch general exception types")]
         private static Color? ParseHexColor(string hex, out string error)
         {
             error = null;
@@ -138,5 +149,43 @@ namespace Spectre.Console.Internal
             error = $"Invalid hex color '#{hex}'.";
             return null;
         }
+
+        [SuppressMessage("Design", "CA1031:Do not catch general exception types")]
+        private static Color? ParseRgbColor(string rgb, out string error)
+        {
+            try
+            {
+                error = null;
+                var normalized = rgb ?? string.Empty;
+                if (normalized.Length >= 3)
+                {
+                    // Trim parenthesises
+                    normalized = normalized.Substring(3).Trim();
+
+                    if (normalized.StartsWith("(", StringComparison.OrdinalIgnoreCase) &&
+                       normalized.EndsWith(")", StringComparison.OrdinalIgnoreCase))
+                    {
+                        normalized = normalized.Trim('(').Trim(')');
+
+                        var parts = normalized.Split(new[] { ',' }, StringSplitOptions.RemoveEmptyEntries);
+                        if (parts.Length == 3)
+                        {
+                            return new Color(
+                                (byte)Convert.ToInt32(parts[0], CultureInfo.InvariantCulture),
+                                (byte)Convert.ToInt32(parts[1], CultureInfo.InvariantCulture),
+                                (byte)Convert.ToInt32(parts[2], CultureInfo.InvariantCulture));
+                        }
+                    }
+                }
+            }
+            catch (Exception ex)
+            {
+                error = $"Invalid RGB color '{rgb}'. {ex.Message}";
+                return null;
+            }
+
+            error = $"Invalid RGB color '{rgb}'.";
+            return null;
+        }
     }
 }