Add color presets. (#725)

* Add DiscordColors struct

* Moved presets to Discord.Color

src/Discord.Net.Core/Entities/Roles/Color.cs

@@ -8,6 +8,46 @@ namespace Discord
     {
         /// <summary> Gets the default user color value. </summary>
         public static readonly Color Default = new Color(0);
+        /// <summary> Gets the teal color value </summary>
+        public static readonly Color Teal = new Color(0x1ABC9C);
+        /// <summary> Gets the dark teal color value </summary>
+        public static readonly Color DarkTeal = new Color(0x11806A);
+        /// <summary> Gets the green color value </summary>
+        public static readonly Color Green = new Color(0x2ECC71);
+        /// <summary> Gets the dark green color value </summary>
+        public static readonly Color DarkGreen = new Color(0x1F8B4C);
+        /// <summary> Gets the blue color value </summary>
+        public static readonly Color Blue = new Color(0x3498DB);
+        /// <summary> Gets the dark blue color value </summary>
+        public static readonly Color DarkBlue = new Color(0x206694);
+        /// <summary> Gets the purple color value </summary>
+        public static readonly Color Purple = new Color(0x9B59B6);
+        /// <summary> Gets the dark purple color value </summary>
+        public static readonly Color DarkPurple = new Color(0x71368A);
+        /// <summary> Gets the magenta color value </summary>
+        public static readonly Color Magenta = new Color(0xE91E63);
+        /// <summary> Gets the dark magenta color value </summary>
+        public static readonly Color DarkMagenta = new Color(0xAD1457);
+        /// <summary> Gets the gold color value </summary>
+        public static readonly Color Gold = new Color(0xF1C40F);
+        /// <summary> Gets the light orange color value </summary>
+        public static readonly Color LightOrange = new Color(0xC27C0E);
+        /// <summary> Gets the orange color value </summary>
+        public static readonly Color Orange = new Color(0xE67E22);
+        /// <summary> Gets the dark orange color value </summary>
+        public static readonly Color DarkOrange = new Color(0xA84300);
+        /// <summary> Gets the red color value </summary>
+        public static readonly Color Red = new Color(0xE74C3C);
+        /// <summary> Gets the dark red color value </summary>
+        public static readonly Color DarkRed = new Color(0x992D22); 
+        /// <summary> Gets the light grey color value </summary>
+        public static readonly Color LightGrey = new Color(0x979C9F);
+        /// <summary> Gets the lighter grey color value </summary>
+        public static readonly Color LighterGrey = new Color(0x95A5A6);
+        /// <summary> Gets the dark grey color value </summary>
+        public static readonly Color DarkGrey = new Color(0x607D8B);
+        /// <summary> Gets the darker grey color value </summary>
+        public static readonly Color DarkerGrey = new Color(0x546E7A);
 
         /// <summary> Gets the encoded value for this color. </summary>
         public uint RawValue { get; }