The AutoColor feature by default only operates on cells that contain numbers or formulas. The cell containing the sum bar itself does not contain any data, so AutoColor does not categorize this as an input. There is an option setting to expand AutoColor operation to non-numeric cells. If you go to FastTrack > User Settings, on the General Tab you'll see checkboxes for "AutoColor cells on entry" and "AutoColor text". If you check both of them, then cells containing textual data--including sum bars--will be treated as inputs.
Note also that once you've checked the "AutoColor text" box, you can turn off AutoColor on entry and the other AutoColor operations (AutoColor selection, AutoColor sheet) will still color text as inputs.