In the recent years, two-color bricks have been showing up in LEGO sets more and more. What most people don’t know is that LEGO has actually been doing two-tone bricks unintentionally for years. It’s not often when you find one in a set, they tend to be weeded out in the quality control phase. Every once in a while we’d find them in the Model Shop at LEGOLAND California, but that is because we use them in such high quantities. Also, a lot of older glued models in the park actually have the marbled bricks as the interior bracing. Since LEGO didn’t sell them, Model Shops would use the “less than perfect” bricks where they can’t be seen by the public. I should know, I’ve seen them in quite a few LEGO models in my time.
So, how do these “unintended” bricks come about? When LEGO changes the color used in a machine injection mold they don’t clean them completely out, they just pour in the new color and let the old color get cycled through. Its during that small amount of time when the old color is still there and the new color has been added that these marbled bricks are born. xbrickmaker has a pretty good amount of them. Look in your collection and see if you have any.