DisabledTitleColorEffect.cs

// In your Forms shared code

using System;
using Xamarin.Forms;

namespace FormsPlayground
{
    public class DisabledTitleColorEffect : RoutingEffect
    {
        string _disabledColor;
        public string DisabledColor
        {
            get
            {
                return _disabledColor;
            }
            set
            {
                _disabledColor = value;
            }
        }

        public DisabledTitleColorEffect() : base ("FormsPlayground.DisabledTitleColorEffect")
        {
            
        }
    }
}

DisabledTitleColorEffect_iOS.cs

// In your iOS project

using System;
using System.Diagnostics;
using System.Linq;
using UIKit;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;

[assembly:ResolutionGroupName ("FormsPlayground")]
[assembly: ExportEffect (typeof (FormsPlayground.iOS.DisabledTitleColorEffect), "DisabledTitleColorEffect")]
namespace FormsPlayground.iOS
{
    public class DisabledTitleColorEffect : PlatformEffect
    {
        protected override void OnAttached ()
        {
            try {
                var effect = (FormsPlayground.DisabledTitleColorEffect)Element.Effects.FirstOrDefault (e => e is FormsPlayground.DisabledTitleColorEffect);
                var btn = (UIButton)Control;
                btn.SetTitleColor(Color.FromHex(effect.DisabledColor).ToUIColor(), UIControlState.Disabled);

            } catch (Exception ex) {
                Debug.WriteLine ("Cannot set property on attached control. Error: ", ex.Message);
            }
        }

        protected override void OnDetached ()
        {
            
        }
    }


}

MyPage.xaml

<?xml version="1.0" encoding="utf-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" 
	xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
	xmlns:local="clr-namespace:FormsPlayground" x:Class="FormsPlayground.FormsPlaygroundPage">
<!-- other things -->
  <Button x:Name="btnForgotPassword" 
			BackgroundColor="Transparent" Text="other text" TextColor="#80868C" Clicked="Handle_Clicked"
			VerticalOptions="Start">
				<Button.Effects>
					<local:DisabledTitleColorEffect DisabledColor="#CC0000"/>
				</Button.Effects>
			</Button>
 <!-- other things -->
</ContentPage>

This was actually very helpful, thanks very much! :)