How do I change the font color on certain text within a cell

You have to be in edit mode in that cell - either click in the formula bar, hit F2, or double-click the cell. Then select the desired text to color. You'll see a pop-up menu with formatting options. If you do not see the pop-up menu, with the text selected, navigate to the Home tab and format from there. You could also right-click the selected text.

There is no way to conditionally format specific text parts of a cell, if that's what you're looking for.