Entre col y col metió una lechuga

Entre col y col metió una lechuga
 

It means that deceivers always find a way to deceive.
ENGLISH?

I found references to 'entre col y col, lechuga' (with varied punctuation) on the Internet. Sometimes the last word was omitted or changed. In every instance I looked at, this refrán meant that some variety was added/inserted, e.g., a professor throwing in a couple of jokes during a lengthy lecture.

If this is a correct interpretation, variety is the spice of life would be a good equivalent.

We sometimes use the verb pepper to mean the same thing. It means to make something lively and vivid using wit (or abusive language). For example:
The long lecture was peppered with jokes, so it was somewhat tolerable.

I suppose you´re right but I always heard it in the context that sb takes the slightest chance to deceive you, because to put a lettuce among a thousand cabbages is not easy to realize.

Entre col y col, lechuga
El refrán advierte especialmente sobre la necesidad de variar las acciones para evitar el cansancio y el aburrimiento por la monotonía. Al mismo tiempo, es de común aplicación a los discursos e intervenciones orales, puesto que aconseja alternar el meollo de la cuestión con anécdotas, dichos o breves historias ilustrativas. Otro significado alude al hecho de que, en ocasiones, nos sucede alguna cosa buena en medio de muchas desgracias. La lechuga, por ser tierna y jugosa, destaca entre las coles, que son más duras y sosas.

In English?

Rusty answered you already. :)