It is an old wives tale that colour can determine contamination. Bacteria will grow and multiply whenever it has the hospitable conditions to do so. No matter the appearance of the mold, it is contamination. The cake has no mold on it coming out of the kill step (oven). Checking product before mold is visible, is key to also determining where and when the contamination occurs. Then controls can be implemented and monitored.
Surface mold almost always has to do with hygienic conditions in handling. It could be due to touch or even in the air. Gloves protect our hands, but if the glove becomes contaminated you spread it around with the glove. If fans are blowing cool air in, the area or on the product, any contamination in the air, dirty fan, etc. will blow on to the product.
Ensuring the product cools sufficiently before packaging is also important. The product surface may be normally dry enough that bacteria does not grow, but if packed to warm, moisture will condensate and provide bacteria the ability to multiply.
If mold is on the interior crumb, before cutting or breaking open, then the internal temperature was not hot enough in baking. The kill step was not completed. If the interior crumb grows mold after cutting or breaking open, then there is most likely contact contamination.
