Well, that is not a challenge to the theory but rather an opinion that it is wrong.
Here is how it works. Coral snakes are older than milk snakes, meaning they evolved before milk snakes did. Coral snakes, for some reason that is not that important for this discussion but which probably had to do with red being a common warning color in the natural world, have red/white/black bands.
The ancestors of milk snakes immigrated into the same habitat as corals, so any pattern similarity between them would be recognized by predators. For example, Lampropeltis tend to have bands and can have the same colors as corals. So, any milk snake ancestors that had colored bands would be more "scary" to predators because they look more similar to corals than those milk snake ancestors that did not look very much like colors.
The milks that look more like corals are preyed upon less, and therefore leave more offspring around. Since color/pattern are heritable the offspring of the milks preyed upon less that have the bands look like there parents. But not perfectly so, some look less like them some more, some have better bands and more defined brighter colors some less. Then, it goes again. The milks that look more like corals are preyed upon less, and therefore leave more offspring around. The cycle starts again.
Your right, there is no "intelligence" that is looking into cells or down upon the snakes patterning them. But natural selection does act as a filter, allowing some to survive while others not too. This "selects" those that are most similar to corals to survive and reproduce. Leading to mimicry. As expected, when species like corals and milks stop living in the same range, over time the mimicry breaks down.....
Here is a cool Nature paper about it:
http://www.nature.com/nature/journal/v451/n7182/full/nature06532.html
Best
Vinny
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“There is a grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that whilst this planet has gone on cycling according to the fixed laws of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.” -C. Darwin, 1859
Natural Selection Reptiles