Some arborvitae have yellow new growth in the spring. It is considered a beautiful part of the spring garden. If your plants have never done this before they could need fertilizing as lack of nitrogen can show up as yellow foliage. From the image you sent it appears the shrubs look healthy. I would wait and see if the plants continue to grow and thrive. That will be your cue to not worry about it. Keep growing, Marianne Binetti