Why will Text Edit and other apps prevent me from choosing text colors?
Do you save your files as .txt (i.e., plain text)? In this case you cannot change colors, because .txt cannot store them, only information that a .txt file can hold are ASCII/UTF-8 characters, no particular formatting.

But you can choose colors if you convert your text into "Rich text" (.rtf file).
To do so, click on Format in the Menubar of TextEdit, and select Make Rich Text. Alternatively, you could also press the shortcut shift+command+T in TextEdit.