Either you hit the format text icon and it was red while you were in that
cell, or possibly it has conditional formatting to change the font color when
a certain phrase is in the cell.
Another possibility is that it wasn't interpreted as text and was
interpreted as a currency value and was formatted to show red for negative
values?
You can try simply clicking the font color icon and choosing Automatic or
the black color and see if that doesn't fix it. If it does not, most likely
conditional formatting has been applied to the cell. If that is the case,
then you can choose the cell and use Edit | Clear and the Formats option to
clear it up (which will also clear it up if it was just formatted red, not
even with conditional formatting).
