A given hen will lay eggs of just one basic color, either white or light brown or medium brown (or whatever.) So I expect either light brown eggs all the time, or white eggs all the time, not switching back and forth between them.
The shade of brown can change a bit, so a chicken who lays "light brown" eggs will have some be a little darker or a little lighter than others.
And a hen who lays any shade of brown eggs can, on rare occasions, lay an entirely white egg (as if her body forgot to put the brown "paint" on the outside of the egg.)