Usually when I see colons, I see them used for a list. For example, I need to get tons of groceries: apples, milk, bread, potatoes, corn, etc. I decided to find out the other meanings to using a colon in the English language.
In the website, https://owl.english.purdue.edu/engagement/2/1/44/, I found out that a colon can also be used after an independent clause when introducing a quotation.
Ex. My teacher’s remark on my final essay was very complimentary: “This essay coherently analyzes musical trends of the late 20th century.”
The other reason a colon may be used is when it is in between two independent clauses and one would want to emphasize the second clause. In the above image, it says "Do Nothing Congress: Just Leave," I believe the reporter wanted to use this colon in particular, to emphasize the second clause. Without reading the actual article, one could assume that Congress is doing nothing, therefore they could just leave. The words "just leave" help make more of a statement to the readers and I think this was the reason the reporter used the colon.