StrokeColorAnimation.cs

Line line = new Line()
{
    Stroke = new SolidColorBrush(Colors.Blue),
    StrokeThickness = 3,
    X1 = 0,
    X2 = 300,
    Y1 = 0,
    Y2 = 200
};

grid.Children.Add(line);

ColorAnimation animation = new ColorAnimation(Colors.Red, new Duration(new TimeSpan(0, 0, 3)));
line.Stroke.BeginAnimation(SolidColorBrush.ColorProperty, animation);