Sure. Set the color of the text for the field to yellow, then go to the Format tab for the field, select the 'Number' format, and for "Negative Number Style" check the "Use Red Text" checkbox. (This is under Acrobat v7, it should be the same for whatever version you are using)
It's a good thing that you want it to be red, cause that's the only choice you have.
Thanks very helpful-works great