Example for using basic, shared resources in xaml. Put this in App.xaml.

LoginPage.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="Myproject.Pages.LoginPage"
             Padding="25, 20, 25, 10"
             BackgroundColor="{StaticResource defaultBackgroundColor}"
             Title="Sign In">
  <ContentPage.Content>
    <StackLayout Padding="10,0,10,20" VerticalOptions="FillAndExpand">
      <StackLayout x:Name="contentLayout" VerticalOptions="CenterAndExpand">
        <Image  Source="PPP_Logo.png"/>
        <Label Text="Email" TextColor="{StaticResource defaultLabelTextColor}" />
        <Entry x:Name="emailEntry" />
        <Label Text="Password" TextColor="{StaticResource defaultLabelTextColor}" />
        <Entry x:Name="passwordEntry" IsPassword="true"/>
        <Button Text="Sign In" TextColor="{StaticResource defaultButtonTextColor}"
        Clicked="OnLoginButtonClicked" BackgroundColor="{StaticResource greenButtonBackgroundColor}"/>
        <ContentView Padding="0, 20">
          <Label Text="Don't have an account?" x:Name="dontHaveAccountLbl" 
          HorizontalTextAlignment="Center" TextColor="{StaticResource defaultLabelTextColor}" FontSize="{StaticResource smallFontSize}" ></Label>
        </ContentView>
      </StackLayout>
    </StackLayout>
  </ContentPage.Content>
</ContentPage>

SharedResource.xaml

<?xml version="1.0" encoding="utf-8" ?>
<Application xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="Myproject.App">
  <Application.Resources>
    <ResourceDictionary>

    <!-- General Color-->
    <Color x:Key="defaultGray">#979797</Color>

    <!--Background Colors-->
    <Color x:Key="defaultBackgroundColor">#FFFFFF</Color>

    <!-- Label Colors-->
    <Color x:Key="defaultLabelTextColor">#979797</Color>
    <Color x:Key="blackLabelTextColor">#000000</Color>
    <Color x:Key="whiteLabelTextColor">#FFFFFF</Color>
    <Color x:Key="blueLabelBackgroundColor">#4A90E2</Color>

    <!-- Entry Colors-->
    <Color x:Key="defaultEntryTextColor">#979797</Color>
    <Color x:Key="defaultEntryPlaceholderColor">#979797</Color>
    <Color x:Key="defaultEntryBackgroundColor">#FFFFFF</Color>

    <!-- Button Colors-->
    <Color x:Key="greenButtonBackgroundColor">#7ED321</Color>
    <Color x:Key="blueButtonBackgroundColor">#3AA2E1</Color>
    <Color x:Key="googleButtonBackgroundColor">#DD4B39</Color>
    <Color x:Key="facebookButtonBackgroundColor">#3B5999</Color>
    <Color x:Key="cancelButtonBackgroundColor">#E40613</Color>
    <Color x:Key="defaultButtonTextColor">#FFFFFF</Color>

    <!-- Font Sizes-->
    <x:Double x:Key="xlargeFontSize">30</x:Double>
    <x:Double x:Key="largeFontSize">24</x:Double>
    <x:Double x:Key="smallFontSize">12</x:Double>
    <x:Double x:Key="xsmallFontSize">10</x:Double>

    </ResourceDictionary>
  </Application.Resources>
</Application>