In many parts of the world , the flow of traffic is controlled in part by traffic lights . Typically , they have three colors : green , yellow , and red . Green means go . Yellow means proceed with care and be prepared to stop . And red means stop . But these colored lights are not only used to control traffic . When it comes to red and green , we also use those two in our English expressions . The most common is the green light . “Greenlight ” means to have permission to go ahead with a project . In English , we often turn verbs into nouns and nouns into verbs . For example , sometimes we use “go-ahead ” as a noun . If you have been given a go-ahead to proceed with a project , you have been given the greenlight . Or in other words , you have been green-lighted , to use the past tense . However , we would not say that we have been “go-aheaded .” Making that past tense is simply not correct . You have been given consent . A greenlight is simply permission to get started ! This term is especially common in the film and theater industry . Producing films and plays takes a lot of resources . For example , the company producing my friend's film refused to green-light production until he did a big rewrite on his script . So , when are you starting your new project ? As soon as I get the greenlight , I can take the first step . The boss gave you the go-ahead in the meeting yesterday . No , she didn't . Yes , she did . I saw her . She nodded her head ! That's body language . For all I know , she could've been tired and her head dropped forward slightly . I've known her for years and that is definitely how she greenlights a project . In fact , just last week she greenlighted a bigger project than yours with just a wink . That's a shockingly subtle way to authorize a project . But also smart . If it doesn't work out , she can say she never gave consent . Well , my project needs a lot of resources . So , I'm going to need more than a wink and a nod , before I proceed . What more could you need ? Oh I don't know … maybe something in writing … like an email ? Now , a red light used as a traffic signal means “stop .” This is also true in conversation . It means an order or directive to stop an action or project . During the pandemic , many companies put a red light on all unnecessary costs . Although , this usage is less common than greenlight . A red light can also mean a signal of danger or a warning . For example , always being late for work could be a red light for a larger problem . Used this way as a warning , we could also say “red flag .” There is another way we use “red light .” Sometimes we combine it with the word “district .” In a red light district adult entertainment is sold .