How to interpret the electrical information on a G1000 T206H? I have a T206H and The POH says:


  Current indication (AMPS) for both the main and standby batteries is
  provided at the bottom of the EIS bar (along the left margin of the
  MFD or PFD), labeled M BATT S. Main battery current is numerically
  displayed below the M. Main battery current greater than -1.5 amps is
  shown in white. Standby battery current is displayed numerically below
  the S. A positive current value (shown in white) indicates that the
  battery is charging. A negative current value (shown in amber)
  indicates that the battery is discharging. In the event the alternator
  is not functioning or the electrical load exceeds the output of the
  alternator, the main battery ammeter indicates the main battery
  discharge rate.


The part that confuses me is the one that says:


  Main battery current greater than -1.5 amps is shown in white.


Does that means that when the number is negative but greater than -1.5 it is OK, because it is in white?

And after 40 minutes of flight in normal daytime operations I have a -1.0 in the M BATT amps but the M BUS volts remain 28.3. What does this mean?
You've actually answered your own question back there:


  A negative current value (shown in amber) indicates that the battery is discharging. In the event the alternator is not functioning or the electrical load exceeds the output of the alternator, the main battery ammeter indicates the main battery discharge rate


The 28.3 Volts reading is the electric energy charge supplied by the aircraft alternator (60-Amp alternator is standard and a 95-amp one is optional) through the main bus, whereas the M amp reading is what's happening to the main battery. In the situation you've described, you've got so many electrically-powered systems running, that the alternator, though operating normally, is unable to supply them all, so additional power is drawn from the battery.

The 206 PIM states that the Main battery Ammeter is color-coded white for values greater than -1.5 Amps, meaning that Cessna is not concerned with battery drains of 1.5 Amps or less (i.e. readings between -1.5 and 0). If the discharge rate is greater than 1.5 Amps (reading lower than -1.5), then the reading will be color-coded yellow. See below for an example of this.