The latest Technical preview builds of Windows 10 (build 10547 onwards) received a very interesting treatment for taskbar app icons of several built-in apps like Store, Outlook Mail, Outlook Calendar or Photos app. If you look closely, you can see that not only are the app icons without any coloful square outline (as default for WinRT apps), but they also have several areas filled by the system's accent color.
I was wondering how this can be achieved. The transparent color is used to make the "transparent" portions of the icon, but the colored fill? I thought that it must be some kind of "default" color that will be replaced by accent color. And as it turned out, I was right! 
If you look at the app assets of the Photos app, you can see a surprisingly large number of different icons for "AppList" or Square 44x44 logo. The normal icons for this kind of icon with appropriate scaling are there, but we can also are the ones with ".targetsize-" suffix. These are offered for entry only for four different sizes - 256, 48, 24 and 16 in Visual Studio. But you can apparently offer the system much more resolutions for even more granular scaling - 100, 96, 80, 64, 40, 32 or 20. But this is where things get even more interesting! There are more different suffixes the Photos app uses!
- contrast-black_targetsize- - white and transparent icon (for high-contrast accessibility setting)
- contrast-white_targetsize- - black and transparent icon (for high-contrast accessibility setting)
- targetsize-_altform-colorize - accent color colorized app icon for taskbar, this is what we are looking for!
- targetsize-_altform-unplated_contrast-white - app icon for taskbar with transparency for high-contrast
- targetsize-_altform-unplated - app icon for taskbar with transparency
- targetsize-_altform-fullcolor - icons used in File Explorer for associated files, this is a full-colored icon
This is a lot of undocumented file suffixes :-) . But the one we are interested here is targetsize-_altform-colorize . This image contains transparent portions for areas of the icon, that are ... well, transparent, any other colors for areas that are visible and finally full black areas for those parts that should be replaced by the system's accent color! Although the official resources and documentation is so far quite lacking, any developer can use these special icon types in their app right now. Just include the surplus images with the correct naming (same as Square 44x44 Logo) in your app's assets and all will work as expected. This gives quite a big creative opportunities for developers to make their app look even more alive on Windows 10.
Sample project
You can download a sample project with colorized taskbar icon here. The project is basically the default blank Windows 10 application with just one additional image file in the Assets folder - Square44x44Logo.targetsize-64_altform-colorize.png. This one file does all the magic! Once you run the application, indeed - you will find yourself looking at a very nice colorized application icon on your taskbar. And once you change the system theme, the icon will change as well :-) . 