In at least one case, Excel will change the “Automatic” font color from black to white for better contrast if the background fill color is too dark. One may observe this by clicking on a Worksheet tab and selecting a darker Tab Color. The default “Automatic” black font will change to white.
I use this example to explain a “good practice” when formatting cells – especially for Column or Row Headings. Keeping the default black-colored font when using a darker fill in a cell often will make the text hard to read on a computer’s screen and upon a printed document. To compensate, a few clients have made the font Bold and Enlarged in order to increase the contrast against a darkly-shaded fill; the result is not always satisfactory. However, white-colored font against a darker cell background does not always translate well to a printed page. Be sure to print a draft of the spreadsheet or report to ensure that a white-font, dark fill-color looks as good on paper as it does on the screen.
Therefore, I usually advise my clients to stick with the default “Automatic” black-colored font against either a white or lighter shaded background fill color.
Quick Note: There is a difference between the color "White" and the "No Fill" Option when selecting cell background colors. Go to a blank cell and choose white as the fill color. Click out of cell and notice that the cell's borders "disappear." Highlight the cell again and change the fill option to "No Fill"; the cell's borders will reappear. This may be the reason that you see missing gridlines on a spreadsheet that you inherited.