[MattD]

Take a look at DriverStation::GetAlliance.

From DriverStation.cpp:

DriverStation::Alliance DriverStation::GetAlliance()
{
	if (m_controlData->dsID_Alliance == 'R') return kRed;
	if (m_controlData->dsID_Alliance == 'B') return kBlue;
	wpi_assert(false);
	return kInvalid;
}

If you look in NetworkCommunication/FRCComm.h, it seems that both alliance and position information is available in the control data packets.

struct FRCControlData{
...
	char dsID_Alliance;
	char dsID_Position;
...
};

Assuming this works properly, you should be able to use this on the field to determine your alliance color without a physical switch.