Lighter or yellowing leaves are caused by a number of reasons. It could be a result of water stress, lack of nitrogen or mineral deficiency. Are you over/under watering it and feeding the palm regularly? A good balanced NPK fertiliser with trace elements will do.

Hi, did you have the palms in the ground outside over the winter?, if they're Phoenix species, such as canary island date palm, Phoenix canariensis, or the true ate palm, Phoenix dactylifera etc, then they will probably be dieing off, {they die very slowly},as they're frost tender, and if left outside in winter in the UK, without a lot of protection, they will be killed off, Derek.