C# Simple Gradient Class

using Zaczero;
(..)

var gradient = new Gradient();

gradient.Add(new GradientColor(0.0, "#ED4264"));
gradient.Add(new GradientColor(0.4, "#4286fF"));
gradient.Add(new GradientColor(1.0, "#FFEDBC"));

var coolColor = gradient.Calculate(0.65);

using System;
using System.Collections.Generic;
using System.Globalization;

namespace Zaczero
{
    public struct GradientColor : IComparable<GradientColor>
    {
        public double Offset;

        public double R;
        public double G;
        public double B;
        public double A;

        public GradientColor(double offset, double r, double g, double b, double a)
        {
            Offset = offset;

            R = r;
            G = g;
            B = b;
            A = a;
        }

        public GradientColor(double offset, double r, double g, double b)
        {
            Offset = offset;

            R = r;
            G = g;
            B = b;
            A = 1d;
        }

        public GradientColor(double offset, string code)
        {
            const string invalidColorCode = "Invalid color code";

            Offset = offset;

            code = code.TrimStart('#');

            foreach (var c in code)
            {
                if (!char.IsLetterOrDigit(c))
                {
                    throw new ArgumentException(invalidColorCode);
                }
            }

            if (code.Length == 6)
            {
                code += "FF";
            }

            if (code.Length == 8)
            {
                var r = code.Substring(0, 2);
                var g = code.Substring(2, 2);
                var b = code.Substring(4, 2);
                var a = code.Substring(6, 2);

                R = byte.Parse(r, NumberStyles.HexNumber) / 255d;
                G = byte.Parse(g, NumberStyles.HexNumber) / 255d;
                B = byte.Parse(b, NumberStyles.HexNumber) / 255d;
                A = byte.Parse(a, NumberStyles.HexNumber) / 255d;
            }
            else
            {
                throw new ArgumentException(invalidColorCode);
            }
        }

        public int CompareTo(GradientColor other)
        {
            return Offset.CompareTo(other.Offset);
        }
    }

    public class Gradient
    {
        private readonly List<GradientColor> _colors;

        public Gradient()
        {
            _colors = new List<GradientColor>();
        }

        public void Add(GradientColor color)
        {
            _colors.Add(color);
            _colors.Sort();
        }

        public GradientColor Calculate(double offset)
        {
            var colorStart = default(GradientColor);
            var colorEnd = default(GradientColor);

            for (var i = 1; i < _colors.Count; i++)
            {
                if (_colors[i].Offset > offset)
                {
                    colorStart = _colors[i - 1];
                    colorEnd = _colors[i];
                    break;
                }
            }

            var offsetRange = colorEnd.Offset - colorStart.Offset;

            var offsetPercentage = (offset - colorStart.Offset) / offsetRange;
            var offsetPercentageInv = 1 - offsetPercentage;

            return new GradientColor(offset,
                colorStart.R * offsetPercentageInv + colorEnd.R * offsetPercentage,
                colorStart.G * offsetPercentageInv + colorEnd.G * offsetPercentage,
                colorStart.B * offsetPercentageInv + colorEnd.B * offsetPercentage,
                colorStart.A * offsetPercentageInv + colorEnd.A * offsetPercentage);
        }
    }
}