Certified GKH Gardening Expert
This will not be an easy question to answer without some important information. Are these in ground, or in container? What have the temperatures been like near the plants? How often do you feed these? Are these leaves newer growth or old growth? This will indicate what deficiency it could be.
In the meantime this article will help: https://www.gardeningknowhow.com/edible/vegetables/pepper/pepper-yellow-leaves.htm