Hi Dory,

You didn't attach photos to your question, so I'm not sure if your shrubs are nutrient deficient or if they are suffering from winter weather. New growth on boxwood is always light green/chartreuse, so it may also be simply that they are happy and growing with the spring. 

Before you apply any chemicals to your soil, it's best if you do a soil test. A soil test will help you understand if essential nutrients are missing and/or if the pH of the soil is impacting your plants. Here is a link to the soil test website. Once you get the results from a soil test, you can make an educated decision of whether your plants need something like iron. 

You can also reply and attach photos.