Broccoli seedling damage any ideas?

Hi there and welcome to the site

What are they planted in (compost?)

Are they indoors or outdoors?

Whereabouts in the UK are you (perhaps add this to your forum profile)?

The seed has germinated and produced two seed leaves. But there is no chlorophyll in them , they are not green. That suggests there is no nutrient in your compost.

Yorkie: I understand what you saying. The seed has germinated but the elements needed to form chlorophyll are not there so photosynthesis cannot take place.
The compost in the photograph is nothing like that I use for germination, so we will have to wait for BMDave to update us.

Hi, thanks for the replys. It was sown  in a greenhouse in seeding compost, as it's been warm I thought I'd trial putting some outside. I've hardened them off for a week first. It's now outside in a bed which has had a horse manure top dressing in autumn that has been worked in recently. The one in the photo is the only one in the ground, I have some others outside and they are also purple but haven't got the damage.