I would consider septoria leaf spot. Except it would be unlikely if you're using a sterile planting mix and confidently disease-free seeds.

Not a sickness I think, just the sunshine bringing out some reddish spots. Not sure if weather stress causes any changes, maybe cold nights? IMO if they grow healthy they should be fine to eat. Many Butterhead types have reddish or speckled color.

My Buttercrunch below has red markings this year. Sometimes I see this. Maybe depends on the seed source? It also has been unusually cold for May so far, maybe that is a factor. A rabbit also gave it a good trim earlier.

My Pirat Butterhead below with a blend of green to reddish brown

I am confident in the seed mix since it comes from a respectable organic brand. And I think my soil should be good because I did grow spinach there before and it grew without problems. But this is my first time adding compost to the soil, I guess that could expose it to bacteria and fungi.

It has been incredibly cold these past couple of weeks for May. Ranging between -2 and 9 degrees Celsius here in Canada (about 25 and 45F), and it has snowed a few days. So that could be a possibility.

Your lettuce looks amazing, I do love the variations in colours. The buttercrunch's markings look very much like the ones on my lettuce. If you have had the markings before and had no issues eating it I'm guessing it's safe to eat.

Reddish speckling, like that on the newest leaf, is normal on young lettuce. Definitely edible and nothing to worry about.

It's just one of the coloured leaf varieties. Perfectly normal and no problem at all.

It looks like Forellenschluss lettuce to me (Forelle is German for trout).

Thank you everyone. As the weather warmed and the plants began to grow faster the marks started to diminish and the new leaves barely have any marks. It seems like it was either stress or cold related.