Brussel Sprout Plants - help!

First time on Forum and first time ever planted anything - so - planted some Brussel Sprout seeds a while back - they are now around 4 inches high in a cold frame and I hope to plant them out in a couple of weeks.  In the last week the leaves have started turning yellow - not burnt from sun as we havent seen any sun in the recent downpour of drought.  Dont appear to be overwatered - any ideas gratefully received!  Thank you

Are they still in the pots they came in?  They might still be in seed compost which has little or no real nutrients.  If you're not ready to plant them out into the garden I'd pot them on into slightly bigger pots with some decent potting compost - maybe using John Innes No 1 compost which is for pricking out young plants.

Good luck with the Brussels Sprouts and beware of wood pigeons - I've just seen a pair prospecting my veg patch 

Oh, and by the way, when you do plant them out make sure you firm the soil around them really well so that they don't rock around in the wind as they grow, or they will make loose leafy sprouts instead of lovely tight button-like sprouts 

Rachael remove the bottom 2 or 3 leaves, then re- pot in bigger pots with John Innes compost.This happened to me last year and only for an old (experienced) gardener  spoting the problem id have lost the crop. Good luck.