I am the proud owner of a Blue Cochin Hen, a trio of Black Cochins, a trio of Red Frizzle Cochins, a Buff Cochin Hen, a Red Cochin, and 8 Red Frizzle Cochin chicks:)
For egg production you want Rhode Island Red, Red Stars, Gold Comets, Black Stars, Dominiques, or Barred Rock. They are all good layers and are hardy birds.
My guess is one of the Leghorns. It could be that the White Rocks egg suddenly turned white but a chickens egg usally does not change color. Guineas lay medium sized brown eggs so it couldnt be them.
Hope this helps,
TDB