Please note that the bases of the 3 valley ski areas - Valle Nevado, El Colorado and La Parva - are accessed by the T bar from Farellones base but not by foot.
You do not have to pay for a pass to go up the Farellones T bar if you mention you are going to the base of El Colorado. From the base of El Colorado you can buy a pass for El Colorado. You can access Valle Nevado skiing from El Colorado or La Parva and buy the upgrade connecting pass for Valle Nevado. Please note to return from Valle Nevado to El Colorado it is a very steep t bar (not for beginners). It is a chairlift to get back to La Parva from Valle Nevado at the end of the day.
You can ski from El Colorado to La Parva to start the day but must buy a pass from El Colorado and La Parva to do so. At the end of the day you can ski from the top of La Parva across to El Colorado easily. To ski at La Parva from Farellones/El Colorado without buying 2 passes you msut get a drive to La Parva in the morning.