Young leaves turning reddish.

Im not sure Cheops but you might have caused this with the seaweed. Too much is as bad as too little. Corn flowers are field flowers that should grow well in even quite poor, well drained soil.

What's your weather been like? Reddening leaves are also a sign of stress.

We have had here over a week of sweltering weather and as I write this, today is the same - very hot. I have kept all the plants and lawn well watered. Looking at the plants , in particuliar the ladybird poppies and the cornflowers they are all in their final place and growing rapidly. All new growth is green now so I suspect I was feeding them too much as Shushkin says. When I was feeding them they were seedlings going on to small plants. I was just curious as this reddishness was only visible but not holding the plants back. The plants were not stressed Mr Bones as they were never drying out.
Living by the seaside I always have a large trug with seaweed soaking in water which I dip into every fortnight, dilute and use it in a watering can all over the flower and vegetable beds. Each other week they get a diluted feed from the liquid which had grass cuttings steeped in water for several days. The hanging baskets get a feed every day. The reddishness seems to have only occurred with the seedlings as they grew and not larger plants. The benefits of this regular feeding is fairly obvious with blooming more abundant than would normally be expected with some of the lupin spires are over two feet high for example.
Seaweed liquid feed which is commercially sold is in my opinion underestimated and often overlooked in favour of other available liquid feeds. Many years ago when beaches were not protected as they are now I hear tales of farmers on the beaches with their tractors and trailers fighting over the seaweed.