use a form , not a datasheet.
I don't think you can change the color of a field in data sheet. However a form would be better, as ranman256 mentioned.
If you use a form instead you can always change the color of a field, font or object quite easily with a few rows code.
//ThornCode:Me.Object_Name.BackColor = RGB(255, 255, 255) Me.Object_Name.ForeColor = RGB(0, 0, 0)
If it must be in datasheet view, you can use the intrinsic 'Conditional Formatting' tool that is found under the Format tab of the Ribbon (while in design view of your form).