Don't know which version you are using. If it was an older version (one I'm using), on the tool bar, you should see a letter "A". That is your font color. Click on the little triangle will bring up the color chart. Select any color for text by clicking on the colored square. Automatic usually defaults to BLACK color.

So if you want to change the text in any cell, simply select that cell, then click "A" and pick your text color. To change an entire worksheet to BLACK, select the entire sheet (click the square above row 1 and column A), then select "A" and "Automatic".

Hope this help.