New Spanish proverb about red hair (and dogs and stones)

On this blog we have several posts about proverbs concerning red hair:

Russian proverbs about redheads and a decree from Peter the Great  

More ancient proverbs about red hair 

European proverbs about red hair  

A Spanish Proverb About Red Hair

Italian Sayings and Proverbs about Red Hair 

Recently, I came across one more Spanish proverb:

Guárdate de hombre pelirrojo, de perro que no ladra y de piedra redonda

that is, beware of a red-haired man, of a dog that does not bark and of a round stone.

Sometimes, proverbs are not very easy to understand, but this one seems quite clear to me.
A round stone, especially when very smooth, is generally a slippery stone (like those we find in rivers).

 
As for the dog, in Italy we have the proverb can che abbaia non morde, that is, a barking dog does not bite (which is the equivalent of the English "its bark is worse than its bite"), and I found out in Spain they have the same: perro que ladra no muerde or perro ladrador, poco mordedor.

Now, if a barking dog does not bite, the opposite is also true: a dog that does not bark may bite.
So, basically, this proverb (just like the others we have seen) is saying that a red-haired man is someone dangerous, untrustworthy, treacherous, someone who looks harmless (like a quiet dog or a round stone) but eventually is not.