Sparrow's beaks changing colors

[beepbeeps]

Hi! I'm new here so I'm not really sure how to post questions like this so I hope I'm doing it right. Anyways, so I have 2 house sparrows, one boy and one girl, they're both around 6 months old. Recently as in the past week or so I've noticed both of their beaks are turning dark greyish instead of their usual yellow, it's a little more noticeable on the girl because her beak is naturally lighter. the color change started on the part of their beak nearest to their face. Is this normal or something I should be concerned about? Their behavior hasn't changed at all, they've always been very active and hyper. Also i was wondering if this could be related to the seasons changing as it's late autumn. I'm going to attach the best pictures I could get, one from a few weeks ago and one from today. Any information is appreciated!

 

[expressmailtome]

@iamwhoiam may be able to help you.