pack://application:,,,/ResourceFile.xaml Never Works

try:

pack://application:,,,/YOURNAMESPACEHERE/ColorThemes.xaml

E.g.

pack://application:,,,/Themes/ColorThemes.xaml

Can't reproduce your problem. All these variations work fine:

<ResourceDictionary.MergedDictionaries>
    <ResourceDictionary Source="pack://application:,,,/Themes/ColorThemes.xaml"/>
    <ResourceDictionary Source="/Themes/ColorThemes.xaml"/>
    <ResourceDictionary Source="../../Themes/ColorThemes.xaml"/>
</ResourceDictionary.MergedDictionaries>

Note that pack://application:,,, is optional and / refers to the root of the current assembly.

Please provide complete minimal example which reproduces your problem.