<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:ClassLibraryTest">

    <SolidColorBrush x:Key="MyButtonColor" Color="#FF0000FF" />

</ResourceDictionary>

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:MainApplicationTest">

    <SolidColorBrush x:Key="MyButtonColor" Color="#00FF00FF" />

</ResourceDictionary>

var mainAppDictionary = new ResourceDictionary();
mainAppDictionary.Source = new Uri("/using:MainApplicationTest;component/MainApplicationDictionary.xaml", UriKind.RelativeOrAbsolute);
classLibraryTest.SetResourceDictionary(mainAppDictionary);