What happens is that you have the egg base colour, and at the last moment, a "bloom" is added to the egg before it is layed.
The density of the bloom can effect the colour, but it isn't "washable" so it can be removed by the bedding or similar.
One of my pullets has just started laying and the bloom on her eggs is so thick that it makes her pale brown eggs look pink/purple! But the bottom of the egg is always straw patterned.