I'm just about to put my runner beans plants out. It's rare to have a frost in Somerset after the end of May but, if one is forecast, I just put some fleece over the plants at night.
As for the yellowing, sometimes the seed leaves and the first true leaves do start to go a bit yellow, even if everything else is OK. I think it's just that they are a bit worn out. If the whole plant is going yellow it might be that the roots are not able to absorb the nutrients in the stable manure for some reason. Try giving them a bit of liquid feed and see what happens.