why are the leaves on my runner beans turning yellow

You don't say how big the pots are, but as runner beans are deep rooting, vigorous plants, which like lots of moisture at their roots, it is probably that they aren't happy. It could be just a lack of something in the compost (do you feed them too with a fertiliser?). I find with pots, even large ones, it is far too easy to either over water or underwater. You often find that while the compost on the surface appears moist, below a shallow layer the compost is dry as a bone. Soilless composts are notoriously difficult to rehydrate. Alternatively, too frequent watering in dull and cool weather and the roots are waterlogged and the plant dies. Try excavating carefully some of the compost in your pots to a good depth at the edge and see if it is either dry or waterlogged. If dry, stand in a tray of water for a day to rehydrate the compost, if waterlogged, stop watering for a while.