What can I set so that whenever I create a text cell, everything I enter into the text cell, whether plain text or some mathematical notation (Ctrl+9 on the Macintosh) is in a particular color I would like to choose?
You can define a new style "BlueText" derived from the "Text" style.
This a partial answer to an issue raised by the OP in a comment to this answer.
I will work up a step by step explanation as soon as I can find the time. Might be a day or two. Meanwhile, you might look at this answer and this one. Also, you might search this site for "stylesheet". BTW the code I show in the private stylesheet needs to be entered with the cell opened to raw form; i.e., select the cell and press Ctrl+Shift+E to get that view. Press the same key combo again to go back to normal display.