Looks like a nitrogen deficiency as is seen here. There are no other signs of typical citrus problems such as iron manganese deficiency which gives dark veins and lighter leaf area.
Some of this may be the soil mix. I see lots of shredded bark which, when combined with a moist soil, could be taking up nitrogen that the citrus roots would otherwise use.
Think about re potting in a smaller pot size with a potting soil mix that does not use as much bark. Should you choose to do so you can get an idea of how well developed the root system is. Small seedlings in a pot will spend a lot of time growing roots so by down sizing and providing new soil you may get more top growth.
You may wish to invest in a citrus fertilizer but it is still early days for your seedlings and a good soil mix should provide all their needs for many months.
Have a look at the numerous answers on this forum for citrus or this one about typical seedling problems.
Edit: Lauren asks how fast should citrus seedlings grow. They are a tree so seedlings I have grown tend to spend the first year growing roots. With more light they will grow faster.