Hi Barb,
Unfortunately the lane or voie from which a train leaves on any given day can change from day to day, depending on how busy things are. The same train
usually leaves from the same voie each time, but this is not always true. The best way to know which voie it will be leaving from is to refer to the large departures board for each zone at Gare de Lyon, which gives colors and lane letters or numbers. This is explained in the
Gare de Lyon photo tour.
Really, you don't have to worry about not figuring this out. It's really evident how it all works when arriving and there are signs everywhere pointing towards both the blue and yellow platforms. By just arriving a few minutes earlier than you would normally, you'll give yourself enough time to look around and see how it works.
Best regards,
Ben