I've never run into a way to access color information (font or background) for use in cell formulas.
You don't say how the numbers come to have different colored fonts. If it happens to be through formatting (e.g. red for negative numbers), then you might want to check out the CELL function. You might be able to set up a column of CELL-generated flags that SUMIF could use. If the colors are set through conditional formatting, then you should be able to set up a column of flags that mimic the conditonal formatting logic.
If the colors are set some other way, then your best bet might be a Visual Basic macro set to run on entry of numbers into the column.