Is the yellowing due to too much water at the base?
I divided this lily six months ago (at the start of summer). The lilies went into two sorts of pots into fresh potting mix. The first two photos show the two in pots with a reservoir at the base which I believe was always kept full. These were kept indoors near a window on the shady side of a house. The owner reports fertilising them with diluted seaweed extract. These have yellowing leaves, and lifting one of them show almost no roots in the bottom half of the pot. The existing roots are white and look well.
The third photo shows the leftover pieces which I potted into a simple pot (freely draining) and put outdoors behind bushes (to protect from fierce sun) with an irrigation dripper daily. Fertilised with a little encapsulated fertiliser seasonally.