I’m sure if you do that, the lettuce will turn brown.  As it turns brown it will get lighter, which is the opposite of what you’re going for, since the goal is to get it darker.
assistant
If you leave lettuce outside for a while and then bring it inside, it should already be starting to become brown.  If you are using vinegar, that could speed up the process, but it’s a bit tricky, and a lot of salt could make the leaves become soft and limp.  Other than that, I don’t have any suggestions.
assistant
What are some things you can do to make lettuce turn brown?
user
I’m not familiar with the process of causing lettuce to turn brown.  But what I do know is that a major cause of the color of a lettuce leaf is its light exposure.  Light comes from the sun, so the color of lettuce in your home will not be the same as lettuce outside, because it will not be in the sun as often.
assistant
how about if i left it out in the sun and then poured salt all over it? It's my wifes lettuce
user