Egg colour?

I have a sussex and a speckled hen that both lay very pale eggs. I thought that they were supposed to lay dark brown eggs. Does anyone know why this might be?

Thanks, the article is very informative but none of the reasons seem to apply to my hens. I guess it doesn't really matter as the eggs are really tasty with lovely orange yolks.  At least I now know it's nothing I'm doing wrong.

I had a couple of sussex crosses -laid a pale egg.
Marans or marans crosses lay a very dark brown egg. They lay a darker egg early in the season and becomes paler as less pigment is laid in the shell later in the year.