#はじめに
プラットフォームに依存しすぎるのか、PCL (Portable Class Library) には、Colorがありません。
解決法は、CrossMVVMを入れるなり、自作ですかね。
わざわざ、nugetで入れるものでもないので、ちょいと作りました。
拡張メソッドを使っているので、使うときは、名前空間をusingしてください。
PCLでのカラーの扱いは、モデル側では、Uintでカラー情報を持たせて、それをView側のコンバーターで、変換するという戦略が良さそうです。こういう拡張メソッドがあれば苦になりませんし。
ちなみに、KnownColorは、referencesource(http://referencesource.microsoft.com/) からKnownColor(WPF版)を拝借しました。

#ソース

filename

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace MyLib.Draw
{
    public static class sample
    {
        public static void 使い方()
        {
　　　　　　//色名をUintに変換
            uint uintColor = KnownColor.AliceBlue.ToUInt();
　　　　　　//αチャンネルの変更
            uint uintColor2 = KnownColor.AliceBlue.ToColor().SetA(125).ToUInt();

        }
    }

    public struct Color
    {
        public byte A { get; set; }
        public byte R { get; set; }
        public byte G { get; set; }
        public byte B { get; set; }
    }


    public static class ColorExtend
    {
        public static uint ToUInt(this Color color)
        {
            return (uint)((color.A << 24) | (color.R << 16) |
                          (color.G << 8) | (color.B << 0));
        }

        public static Color ToColor(this uint color)
        {
            byte a = (byte)(color >> 24);
            byte r = (byte)(color >> 16);
            byte g = (byte)(color >> 8);
            byte b = (byte)(color >> 0);
            return new Color() { A = a, R = r, G = g, B = b };
        }

        public static Color ToColor(this KnownColor known)
        {
            return ((uint)known).ToColor();
        }
        public static uint ToUInt(this KnownColor known)
        {
            return ((uint)known);
        }

        public static Color SetA(this Color color ,byte a)
        {
            return new Color() { A = a, R = color.R, B = color.B, G = color.G };
        }
    }

    /// <summary>
    /// referencesource(http://referencesource.microsoft.com/) からKnownColor(WPF版)を拝借
    /// </summary>
    public enum KnownColor : uint
    {
        // We've reserved the value "1" as unknown.  If for some odd reason "1" is added to the
        // list, redefined UnknownColor

        AliceBlue = 0xFFF0F8FF,
        AntiqueWhite = 0xFFFAEBD7,
        Aqua = 0xFF00FFFF,
        Aquamarine = 0xFF7FFFD4,
        Azure = 0xFFF0FFFF,
        Beige = 0xFFF5F5DC,
        Bisque = 0xFFFFE4C4,
        Black = 0xFF000000,
        BlanchedAlmond = 0xFFFFEBCD,
        Blue = 0xFF0000FF,
        BlueViolet = 0xFF8A2BE2,
        Brown = 0xFFA52A2A,
        BurlyWood = 0xFFDEB887,
        CadetBlue = 0xFF5F9EA0,
        Chartreuse = 0xFF7FFF00,
        Chocolate = 0xFFD2691E,
        Coral = 0xFFFF7F50,
        CornflowerBlue = 0xFF6495ED,
        Cornsilk = 0xFFFFF8DC,
        Crimson = 0xFFDC143C,
        Cyan = 0xFF00FFFF,
        DarkBlue = 0xFF00008B,
        DarkCyan = 0xFF008B8B,
        DarkGoldenrod = 0xFFB8860B,
        DarkGray = 0xFFA9A9A9,
        DarkGreen = 0xFF006400,
        DarkKhaki = 0xFFBDB76B,
        DarkMagenta = 0xFF8B008B,
        DarkOliveGreen = 0xFF556B2F,
        DarkOrange = 0xFFFF8C00,
        DarkOrchid = 0xFF9932CC,
        DarkRed = 0xFF8B0000,
        DarkSalmon = 0xFFE9967A,
        DarkSeaGreen = 0xFF8FBC8F,
        DarkSlateBlue = 0xFF483D8B,
        DarkSlateGray = 0xFF2F4F4F,
        DarkTurquoise = 0xFF00CED1,
        DarkViolet = 0xFF9400D3,
        DeepPink = 0xFFFF1493,
        DeepSkyBlue = 0xFF00BFFF,
        DimGray = 0xFF696969,
        DodgerBlue = 0xFF1E90FF,
        Firebrick = 0xFFB22222,
        FloralWhite = 0xFFFFFAF0,
        ForestGreen = 0xFF228B22,
        Fuchsia = 0xFFFF00FF,
        Gainsboro = 0xFFDCDCDC,
        GhostWhite = 0xFFF8F8FF,
        Gold = 0xFFFFD700,
        Goldenrod = 0xFFDAA520,
        Gray = 0xFF808080,
        Green = 0xFF008000,
        GreenYellow = 0xFFADFF2F,
        Honeydew = 0xFFF0FFF0,
        HotPink = 0xFFFF69B4,
        IndianRed = 0xFFCD5C5C,
        Indigo = 0xFF4B0082,
        Ivory = 0xFFFFFFF0,
        Khaki = 0xFFF0E68C,
        Lavender = 0xFFE6E6FA,
        LavenderBlush = 0xFFFFF0F5,
        LawnGreen = 0xFF7CFC00,
        LemonChiffon = 0xFFFFFACD,
        LightBlue = 0xFFADD8E6,
        LightCoral = 0xFFF08080,
        LightCyan = 0xFFE0FFFF,
        LightGoldenrodYellow = 0xFFFAFAD2,
        LightGreen = 0xFF90EE90,
        LightGray = 0xFFD3D3D3,
        LightPink = 0xFFFFB6C1,
        LightSalmon = 0xFFFFA07A,
        LightSeaGreen = 0xFF20B2AA,
        LightSkyBlue = 0xFF87CEFA,
        LightSlateGray = 0xFF778899,
        LightSteelBlue = 0xFFB0C4DE,
        LightYellow = 0xFFFFFFE0,
        Lime = 0xFF00FF00,
        LimeGreen = 0xFF32CD32,
        Linen = 0xFFFAF0E6,
        Magenta = 0xFFFF00FF,
        Maroon = 0xFF800000,
        MediumAquamarine = 0xFF66CDAA,
        MediumBlue = 0xFF0000CD,
        MediumOrchid = 0xFFBA55D3,
        MediumPurple = 0xFF9370DB,
        MediumSeaGreen = 0xFF3CB371,
        MediumSlateBlue = 0xFF7B68EE,
        MediumSpringGreen = 0xFF00FA9A,
        MediumTurquoise = 0xFF48D1CC,
        MediumVioletRed = 0xFFC71585,
        MidnightBlue = 0xFF191970,
        MintCream = 0xFFF5FFFA,
        MistyRose = 0xFFFFE4E1,
        Moccasin = 0xFFFFE4B5,
        NavajoWhite = 0xFFFFDEAD,
        Navy = 0xFF000080,
        OldLace = 0xFFFDF5E6,
        Olive = 0xFF808000,
        OliveDrab = 0xFF6B8E23,
        Orange = 0xFFFFA500,
        OrangeRed = 0xFFFF4500,
        Orchid = 0xFFDA70D6,
        PaleGoldenrod = 0xFFEEE8AA,
        PaleGreen = 0xFF98FB98,
        PaleTurquoise = 0xFFAFEEEE,
        PaleVioletRed = 0xFFDB7093,
        PapayaWhip = 0xFFFFEFD5,
        PeachPuff = 0xFFFFDAB9,
        Peru = 0xFFCD853F,
        Pink = 0xFFFFC0CB,
        Plum = 0xFFDDA0DD,
        PowderBlue = 0xFFB0E0E6,
        Purple = 0xFF800080,
        Red = 0xFFFF0000,
        RosyBrown = 0xFFBC8F8F,
        RoyalBlue = 0xFF4169E1,
        SaddleBrown = 0xFF8B4513,
        Salmon = 0xFFFA8072,
        SandyBrown = 0xFFF4A460,
        SeaGreen = 0xFF2E8B57,
        SeaShell = 0xFFFFF5EE,
        Sienna = 0xFFA0522D,
        Silver = 0xFFC0C0C0,
        SkyBlue = 0xFF87CEEB,
        SlateBlue = 0xFF6A5ACD,
        SlateGray = 0xFF708090,
        Snow = 0xFFFFFAFA,
        SpringGreen = 0xFF00FF7F,
        SteelBlue = 0xFF4682B4,
        Tan = 0xFFD2B48C,
        Teal = 0xFF008080,
        Thistle = 0xFFD8BFD8,
        Tomato = 0xFFFF6347,
        Transparent = 0x00FFFFFF,
        Turquoise = 0xFF40E0D0,
        Violet = 0xFFEE82EE,
        Wheat = 0xFFF5DEB3,
        White = 0xFFFFFFFF,
        WhiteSmoke = 0xFFF5F5F5,
        Yellow = 0xFFFFFF00,
        YellowGreen = 0xFF9ACD32,
        UnknownColor = 0x00000001
    }

}