The Logitech is a 5.1 system. The "orange" plug is more likely to be what is normally pink.
How many output/input sockets does your sound card have?
In the case of my on-board Realtek ALC655 chipset's three output sockets (it is capable of 5.1), by configuring the Sound Effects Manager control panel for 2.1, 4.1 or 5.1, the Line Out and MIC sockets are converted to two-way operation.
Thus for 5.1 output (five satellites and sub-woofer), the Line Out handles the rear channels, the second Line Out (green, stereo output) the front channels and the third (MIC, black socket) deals with Centre and sub-woofer channels).
I suspect this may be the case with your system - have a look at the sound card's Help pages regarding configuration.