I'm not sure if you are north or south of the equator.  Lighting has a direct effect on how and when they lay.  I don't know if your days are getting longer or shorter.  That information could be useful.  If you are south of the equator some hens will start to lay after they finish the molt regardless of whether the days are getting longer or shorter, but some will wait until the days are getting longer.  Temperatures can make a difference in this too.  If you are south of the equator supplemental lighting might help.
The eggs a hen lays can change shade over time.  The base color is set by genetics, either blue or white.  Brown of green is brown laid on over that5 base color.  The longer the hen lays the lighter the egg can become.  Some of my brown or green egg layers can lay a pretty much white or blue egg just before they molt.  But the first eggs after a molt should be as dark as they will ever be. 
It's always possible a hen can have an oops.  The way the coloring works is that the egg spends many hours in the shell gland putting on the shell, then for a brown or green egg the coloring is laid on during the last half hour or so.  If something happens to cause the hen to lay the egg early before that color is added a hen that typically lays a brown egg can lay a white egg. 
If it is a one-off thing and not regular, it's just an oops, no big deal.  If you are sure it is that hen, she laid a brown egg before the molt, and it is a daily thing that might be cause for concern.  Some diseases can cause that and also interfere with chickens laying eggs.  But normally they act sick.  Most of these types of disease can kill some.  If they are acting normal they are probably not sick.  Another possibility is that she may have eaten something that could cause this.  If I remember right if she ate some mold it can cause white eggs.  It is hard to diagnose these things over the internet, you can't see what else is going on. 
I would not panic but would keep an eye on them and see how that are acting.  Sometimes these things just happen and there really isn't a problem or it is something they correct on their own.  If they start dying or acting sick then I'd be concerned.