Yes, it can be pretty stressful when our tots throw us these challenges! It is quite common for them to go through these phases, and as parents, we just want to ensure that they are getting all the nutrients they need. We do our best, and fortunately, most of the time, these phases pass with some patience and creativity!
My tot had a phase where she would only eat foods that were orange--carrots, oranges, sweet potatoes--you get the idea! It was quite cute at first, but it meant that mealtimes became quite boring and one-dimensional. I tried introducing other fruits and veggies, but she was absolutely steadfast in her "orange" requirement. It didn't help that her favorite word during that period was a firm and resolute, 不要(Don't want!) for everything else.
It passed after about two months, thank goodness, but I can certainly relate to the stress of worrying about my tot's nutrition during those times.