paperclippy

"When you use a colon in a sentence, you are NOT supposed to capitalize the word after the colon."

I would consider this a matter of style, not grammar. Style guides seem to differ on this point, and some offer quite complex guidelines. Just a small excerpt:

There is some disagreement among writing reference manuals about when you should capitalize an independent clause following a colon. Most of the manuals advise that when you have more than one sentence in your explanation or when your sentence(s) is a formal quotation, a capital is a good idea. The NYPL Writer's Guide urges consistency within a document; the Chicago Manual of Style says you may begin an independent clause with a lowercase letter unless it's one of those two things (a quotation or more than one sentence). The APA Publication Manual is the most extreme: it advises us to always capitalize an independent clause following a colon. The advice given above is consistent with the Gregg Reference Manual.

Perhaps you are thinking of guidelines for dependent clauses only?