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When using a colon in a sentence ex: My understanding of this letter is as follows:
do you capitalize the next word after the colon?
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Hi,

"If the explanation that follows your colon makes a complete sentence, you may capitalize the first word after the colon. This is a choice unless you’re introducing a rule. Then you must capitalize the first word. Most people capitalize all sentences after a colon to be consistent throughout their writing."

The people who wrote on the link you provided may have excellent grammar and linguistic qualifications, or they may not. I don't know, as is often the case with things you find on the Internet. Anyway, I don't agree with what they have written here. Here are a couple of their examples.

Example: My job as a facilitator requires me to be proficient in three areas: reading, writing, and talking. Starting a sentence with the word 'Example' like this is not good prose. For example, you should never write this way in an essay. Instead, say 'For example,. . .'

What I want to convey in this workshop is simple: Writing is about making choices. Here, I simply disagree with the capitalization of 'writing'.

To me, the flaw in the above rule is that it is written on the assumption that colons separate sentences. They don't. What precedes and follows a colon in prose is all part of the same sentence.

Best wishes, Clive
Comments  
Colon isn't a letter and you don't capitlize the letter preceded by the colon.
Students: We have free audio pronunciation exercises.
"If the explanation that follows your colon makes a complete sentence, you may capitalize the first word after the colon. This is a choice unless you’re introducing a rule. Then you must capitalize the first word. Most people capitalize all sentences after a colon to be consistent throughout their writing."

From http://www.csusm.edu/professionalwriters/Making%20Grammar%20Work%20for%20You.htm
 Clive's reply was promoted to an answer.