They have gone yellow because they have used up all the nutrients that were available in that small cup. Next time either transplant sooner or use larger containers to start your seedlings in. This will also provide for much more growth earlier. As you can see, the roots are well and truly filling that little cup, which slows growth considerably. I am sure they will now power away in their new homes with so much extra room for root growth. A quick tip, although too late for Nancy Lou, next time fill your pots right up to the top, this will allow much more room for root growth and consequently, plant growth, without using any extra space in your grow area. Also, the substrate in the pot will compress down after a while, further limiting root growth as compared to a full pot. Good luck and happy growing!
Hope this helps,... Organoman.