That's because it is the only livery for the CS_B777_PAX.

If you want to add a different airline, you will need to copy the texture.xxx (i.e. texture.ba from the CS_B777-200ER_GE folder) into the CS_B777-PAX folder, then add the ([fltsim.1]) entry into the aircraft.cfg like this:

[fltsim.0]
title=CS 777-200 PAX Captain Paint0
sim=csx772_GE
model=PAX
panel=
sound=
texture=klm
kb_checklists=
kb_reference=
atc_id=PH-BQF
atc_airline=KLM
atc_flight_number=777
atc_heavy=1
ui_manufacturer=Boeing
ui_type=777 Cabin
ui_variation=Passenger cabin
ui_typerole="Commercial Airliner"
ui_createdby="Captain Sim"
engine_type=GE90-94B
description="777 Passenger cabin\n777 Captain 1.7\n(c) 2015 Captain Sim www.captainsim.com"

[fltsim.1]
title=CS 777-200 PAX Captain Paint1
sim=csx772_GE
model=PAX
panel=
sound=
texture=ba
kb_checklists=
kb_reference=
atc_id=G-VIIT
atc_airline=British Airways
atc_flight_number=777
atc_heavy=1
ui_manufacturer=Boeing
ui_type=777 Cabin
ui_variation=Passenger cabin British Airways
ui_typerole="Commercial Airliner"
ui_createdby="Captain Sim"
engine_type=GE90-94B
description="777 Passenger cabin\n777 Captain 1.7\n(c) 2015 Captain Sim www.captainsim.com"

The bold text shows the differences between the KLM entry and the British Airways entry.

Note. ALWAYS BACK UP THE ORIGINAL AIRCRAFT.CFG FILE BEFORE MAKING ANY CHANGES!