How can I change the colors of a stepper for iOS and Android?

This can be done using Effects.

Code

I've created a sample app here: https://github.com/brminnick/CustomStepper

Consuming the Effects in 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" 
    x:Class="CustomStepper.StepperPage"
    xmlns:local="clr-namespace:CustomStepper">
    
    <ContentPage.Content>
        <Stepper
            HorizontalOptions="Center"
            VerticalOptions="Center"
            local:StepperColorEffect.Color="Red"/>
    </ContentPage.Content>
</ContentPage>

Stepper Color Effect

using System.Linq;

using Xamarin.Forms;

namespace CustomStepper
{
    public static class StepperColorEffect
    {
        public static readonly BindableProperty ColorProperty =
            BindableProperty.CreateAttached(nameof(Color),
                typeof(Color),
                typeof(Stepper),
                Color.Gray,
                propertyChanged: OnStepperColorChanged);

        public static Color GetColor(BindableObject view) => (Color)view.GetValue(ColorProperty);

        public static void SetColor(BindableObject view, Color value) => view.SetValue(ColorProperty, value);

        static void OnStepperColorChanged(BindableObject bindable, object oldValue, object newValue) => UpdateEffect(bindable);

        static void UpdateEffect(BindableObject bindable)
        {
            var stepper = (Stepper)bindable:
            RemoveEffect(stepper);
            stepper.Effects.Add(new StepperColorRoutingEffect());
        }

        static void RemoveEffect(Stepper entry)
        {
            var effectToRemoveList = entry.Effects.OfType<StepperColorRoutingEffect>();

            foreach (var entryReturnTypeEffect in effectToRemoveList)
                entry.Effects.Remove(entryReturnTypeEffect);
        }
    }

    class StepperColorRoutingEffect : RoutingEffect
    {
        public StepperColorRoutingEffect() : base("CustomStepper.StepperColorEffect")
        {
        }
    }
}

iOS PlatformEffect

using UIKit;

using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;

using CustomStepper.iOS;

[assembly: ResolutionGroupName("CustomStepper")]
[assembly: ExportEffect(typeof(StepperColorEffect), nameof(StepperColorEffect))]
namespace CustomStepper.iOS
{
    public class StepperColorEffect : PlatformEffect
    {
        protected override void OnAttached()
        {
            if (Element is Stepper element && Control is UIStepper control)
            {
                control.TintColor = CustomStepper.StepperColorEffect.GetColor(element).ToUIColor();
                control.SetIncrementImage(Control.GetIncrementImage(UIControlState.Normal), UIControlState.Normal);
                control.SetDecrementImage(Control.GetDecrementImage(UIControlState.Normal), UIControlState.Normal);
            }
        }

        protected override void OnDetached()
        {
            if (Element is Stepper element && Control is UIStepper control)
            {
                control.TintColor = UIColor.Blue;
                control.SetIncrementImage(Control.GetIncrementImage(UIControlState.Normal), UIControlState.Normal);
                control.SetDecrementImage(Control.GetDecrementImage(UIControlState.Normal), UIControlState.Normal);
            }
        }
    }
}

Android PlatformEffect

using Android.Widget;
using Android.Graphics;

using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;

using CustomStepper.Droid;

[assembly: ResolutionGroupName("CustomStepper")]
[assembly: ExportEffect(typeof(StepperColorEffect), nameof(StepperColorEffect))]
namespace CustomStepper.Droid
{
    public class StepperColorEffect : PlatformEffect
    {
        protected override void OnAttached()
        {
            if (Element is Stepper element && Control is LinearLayout control)
            {
                control.GetChildAt(0).Background.SetColorFilter(CustomStepper.StepperColorEffect.GetColor(element).ToAndroid(), PorterDuff.Mode.Multiply);
                control.GetChildAt(1).Background.SetColorFilter(CustomStepper.StepperColorEffect.GetColor(element).ToAndroid(), PorterDuff.Mode.Multiply);
            }
        }

        protected override void OnDetached()
        {
            if (Element is Stepper element && Control is LinearLayout control)
            {
                control.GetChildAt(0).Background.SetColorFilter(Xamarin.Forms.Color.Gray.ToAndroid(), PorterDuff.Mode.Multiply);
                control.GetChildAt(1).Background.SetColorFilter(Xamarin.Forms.Color.Gray.ToAndroid(), PorterDuff.Mode.Multiply);
            }
        }
    }
}

Screenshots

Android

iOS

How can I change the colors of a stepper for iOS and Android?

In your Xamarin.Android project, you could create a custom vector drawable and use it as a background to achieve the same appearance as the picture you have post above.

Place the button_selector.xml and button_border.xml file in your Android Resources\drawable folder:

button_selector.xml

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
  <item android:drawable="@color/colorAccent" android:state_pressed="true"/>
  <item android:drawable="@color/colorPrimaryDark" android:state_focused="true"/>
  <item android:drawable="@drawable/button_border"/>
</selector>

button_border.xml:

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
       android:shape="rectangle" >

  <solid android:color="#00FFFFFF" />
  <corners android:radius="5dp" />
  <stroke
    android:width="2dp"
    android:color="#FFFFFF" />

</shape>

In your ExtStepperRenderer:

protected override void OnElementChanged(ElementChangedEventArgs<Stepper> e)
{
    base.OnElementChanged(e);
    MyStepper s = Element as MyStepper;

    if (Control != null)
    {
        var button = Control.GetChildAt(0) as Android.Widget.Button;
        button.SetTextColor(s.MyColor.ToAndroid());
        button.SetBackground(ResourcesCompat.GetDrawable(Resources, Resource.Drawable.button_selector, null));
        button.Background.SetColorFilter(s.MyColor.ToAndroid(), PorterDuff.Mode.Multiply);

        var button2 = Control.GetChildAt(1) as Android.Widget.Button;
        button2.SetTextColor(s.MyColor.ToAndroid());
        button2.SetBackground(ResourcesCompat.GetDrawable(Resources, Resource.Drawable.button_selector, null));
        button2.Background.SetColorFilter(s.MyColor.ToAndroid(), PorterDuff.Mode.Multiply);
    }
}

Effect on Android device: