Hi and welcome! :)
The darkened units are part of the unlock/experience system. When you play online, you get experience to unlock new units to be able to make ingame.
Of course, if you are playing single player, you probably don't want you units locked. :) There is a special option to unlock everything. Under Singleplayer click on the "Options" tab, and under "Experimental Settings" uncheck "Enable unlock system". This will allow all units to be buildable ingame. Enjoy!
