I believe that the sentence should read:

"But I had a good reason; the fashion police were after me."

A semi-colon is the appropriate punctuation when connecting two interrelated, yet independent, clauses.

I think the hyphenation is correct, though.

OK, my example wasn't well-chosen, then... how about:

Here's why: You wear polka-dot overalls to work!

Capitalized, right?

According to the Chicago Manual of Style:

A colon introduces an element or a series of elements illustrating or amplifying what has preceded the colon. Between independent clauses it functions much like a semicolon, though more strongly emphasizing sequence. The colon may be used instead of a period to introduce a series of related sentences (as in the fourth example below). Colons are also used in URLs; no space precedes or follows a colon in a URL. For use of the em dash instead of a colon, see 6.88. (A colon should never be immediately followed by a dash: either a colon or a dash alone suffices.)

The study involves three food types: cereals, fruits and vegetables, and fats.
They even relied on a chronological analogy: just as the Year II had overshadowed 1789, so the October Revolution had eclipsed that of February.
Many of the police officers held additional jobs: thirteen of them, for example, moonlighted as security guards.
Henrietta was faced with a hideous choice: Should she reveal what was in the letter and ruin her reputation? Or should she remain silent and compromise the safety of her family?
You should be able to find an archived version of the article at http://www.nytimes.com.

When a colon is used within a sentence, as in the first three examples in 6.63, the first word following the colon is lowercased unless it is a proper name. When a colon introduces two or more sentences (as in the fourth example in 6.63 or the second example in 6.65), or when it introduces a speech in dialogue or an extract (as in the examples in 6.66), the first word following it is capitalized.

So, at least according to this style guide, Robin's use of the colon in the sentence is acceptable; however, his post-colon capitalization is mistaken.

So, when do you cap after a colon? When you're listing a series that includes multiple sentences.

Here's why: you wear polka-dot overalls to work!

or

Here's why: You wear polka-dot overalls to work! And they don't even match the company logo!

are both correct. (At least for the capitalization; I'm not sure either sentence is ideal.)

I agree that the hyphenation is sound.

I also suggest a new game: guess the dialogue immediately preceding Robin/Tim's polka-dot sentence.

My entry: "Robin, I have bad news. The vice president wants to let you go. He thinks you're cheating on us with another clown college."

A-ha. Re: the colon, I have been doing it wrong. Argh... this is going to take a while to unlearn.

I will happily go all LanguageHat/descriptivist on your grammar and say that clarity of communication trumps adherence to axiom. I frequently capitalize independent clauses after colons because that makes sense to me. In any case, neither construction is more or less effective. The Chicago Manual capitalization rule seems like it was written by an insomniac copy editor. It would take me a lifetime of looking at colon-following independent clauses before I could parse that silly two-or-more exception.

Insomnia is a good quality in a copy-editor. :)

1) A simpler rule would be to say that you only cap after a colon when the units that follow it are complete sentences. Or you could say NEVER cap after a colon, and if you positively have to chain sentence-y units together, use semi-colons rather than terminal punctuation. Either is perfectly consistent

2) My take on style guides is that they provide clarity in cases of indecision. It's sort of like the rule that when two cars arrive simultaneously at a four-way stop, the car on the right goes first. Their job is to resolve cruxes decisively, so we don't get caught in indeterminacy.

In most cases, ANY rule that is well-defended by history and usage that provides a clear guideline to writers is perfectly acceptable. Because in the publication chain, SOMEBODY inevitably has to make that call.

Unfortunately, it depends. I've been an editor at two carefully copyedited national magazines—the pair are competitors, even—and the copy desks at each would treat these examples differently.

First example: Magazine 1's rule is that a compound adjective always takes a hyphen.
Magazine 2's rule is more subtle. In general a compound adjective takes a hyphen. However, if the compound adjective is so frequently used that there is no danger of the reader mistaking what is being modified—e.g. "health care costs," the hyphen can be omitted.

Second example: Magazine 1's rule is by-the-book Chicago, as Tim explains above. Magazine 2 capitalizes a complete sentence after the colon (which I prefer).

Anyway, I side with Matt: Always be clear.

The secret of all punctuation and grammar is that it depends on which style book you're using. It's slightly less important that a writer gets it "right" than that an individual publication does it the same way every time.

Louis Menand in The New Yorker (reviewing the 15th Edition of the Chicago Manual of Style) still has it right:

Some people will complain that the new ___SIMPLE_HTML_DOM__VOKU__AMP____#128;œChicago Manual” is too long. These people do not understand the nature of style. There is, if not a right way, a best way to do every single thing, down to the proverbial dotting of the “i.” Relativism is fine for the big moral questions, where we can never know for sure; but in arbitrary realms like form and usage even small doses of relativism are lethal. The “Manual” is not too long. It is not long enough. It will never be long enough. The perfect manual of style would be like the perfect map of the world: exactly coterminous with its subject, containing a rule for every word of every sentence. We would need an extra universe to accommodate it. It would be worth it.

Yeah, agree re: consistency. The mistakes that make me go "doh" are the ones that involve doing the same thing two different ways. Or not feeling like I have a firm enough grasp on the distinctions, e.g. the colon-capitalization question.

One of my favorite things to behold is the bloggers who have developed a personal style that is at once a) absolutely off the map -- all lowercase, weird misspellings, contractions, etc. -- and b) absolutely consistent. Hipster Runoff is a prime example. He has basically developed his own HRO Style Guide on the fly. It's a wonder to behold. (Especially his use of quote-marks. It's bizarre but purposeful. And again: largely consistent.)

If the freaky characters (they're quotation marks that didn't translate) make the paragraph I quoted unreadable, the full article is available here: http://www.newyorker.com/archive/2003/10/06/031006crbo_books1