me dwarf beans are looking yellow!

Probably the cool weather we've had for the last couple of weeks. When and if it warms up give them a liquid feed and they should perk up. Watch out for ground frost and cover them with fleece if its forecast as its still possible until early June.

if they are out in the big wide world then its the cold nights if they are indoors and wrapped up warm its lack of nutrients....

Runners and French are warm weather buddies - mine are tucked up in the greenhouse and wont go out until the last week of may IF i think its ok... by all means take a chance but always have backups.  My expereince is you gain very little by getting them out early, they sit and sulk and the stuff you sow now or plant out at the end of the month soon catches up