Is it possible to point one Color resource to another Color resource in Xamarin.Forms?

You can use x:Static in tandem with a static class in order to directly reference those colors by name. This has the benefits of keeping the colors centralized into one class and minimizing the amount of XAML.

namespace ResourceColors
{
    public static class Colors
    {
        public static Color Slate = Color.FromHex("#404040");
    }
}

<?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:ResourceColors;assembly=ResourceColors" x:Class="ResourceColors.PageOne">
    <ContentPage.Resources>
        <ResourceDictionary>
            <Color x:Key="Blue">#458623</Color>
        </ResourceDictionary>
    </ContentPage.Resources>
    <ContentPage.Content>
        <StackLayout HorizontalOptions="CenterAndExpand" VerticalOptions="CenterAndExpand">
            <Label Text="Test" TextColor="{x:Static local:Colors.Slate}" />
        </StackLayout>
    </ContentPage.Content>
</ContentPage>