Silver cuckoo marans egg color change?

Hi everyone. I have a perhaps silly question/concern. I have 4 chickens, hatched on April 27th 2015, they are silver cuckoo marans, cream legbar, black jersey giant and dominique. The marans laid her first egg at 18 weeks and 5 days or so, it wasnt the chocolate color I hopped for but kinda expected it since they came from mypetchicken. The eggs darkened some over the first week and she was laying about 5 tiny eggs (about 34 grams) a week. She laid 16 eggs and than took a break, maybe 2 weeks. She started to lay again 3 days ago but her eggs are so much lighter in color. The dominique started to lay week and a half ago. I am now getting 2 light brown eggs a day. I am positive its the dominque and marans laying them, I am just baffled as to why did the marans started to lay eggs that are so much lighter? I am thinking she might have been sick, I've noticed water dripping from her nostril last week but only for few days, she wasn't wheezing but she did "sneeze" a little bit and kept shaking her head some. She's been pretty mean to the other pullets lately - more than usual, especially the jersey giant and the dominique, they both have dark spots on their combs from the pecking. I checked them for lice and mites, cleaned the coop(pine shavings and hay for bedding and sand for the run). They are fed nature's best organic layer pellets 16% mixed with purina flock raiser crumbles. Oyster shell free choice is also available. Freeze-dried mealworms few times a week as a treat. They free range from noon till sunset almost every day. I also noticed the jersey giant kept one of her eyes closed a lot today, it didnt look swollen and looked healthy. I did a bit of googling and am thinking maybe they have infectious bronchitis? Would that affect the egg color of the marans? They all act normal, eating and drinking as usual, running around. Last question, they don't seem underweight to me but the breastbone is very prominent on all 4, is it because they are still young or are they not eating enough?
Thank you for reading and for replies

Chickens usually lay the darkest eggs at the beginning of the laying season then they get lighter, some of my dark egg layer do as yours is, laying sporadically, stopping and starting, as well as the eggs getting lighter and darker, so I think what you are seeing is normal and you should expect things to be unexpected with your maran.

So apparently I was wrong. The light colored egg is from the black jersey giant. I caught her in action the other day. I just assumed it couldn't be her because I've read that they don't lay until much later. She started laying around 24 weeks even before the cream legbar which laid her first today. So the marans is still not laying. I hope she starts again soon and is not sick. She is still mean to the other hens.
These are the marans eggs


And these are: left BJG, right dominique, bottom cream legbar

Glad you figured it out, there's a general way of things about chickens, and then there's how they do things, always expect the unexpected, nice looking eggs.