... we're wandering the Plains of Pedantry (and at the welcome risk of being burned in olive oil with a touch of garlic and rosemary), the devil is ever in the detail:
"...and we learn that Google follows the usage of all civilised persons: it instructs devs not to capitalise the first word after the colon."
Hmmm. So they would require a lower case version of the word I (referring to the personal pronoun, and yes - I is a word in this context)? Or a lower case R for Richard if Richard were the first of a list of names? I sincerely hope not, and hope further that no 'civilised person', never mind 'all' of them would either.
And now, just for fun, we can perhaps proceed to the question of whether it is grammatically correct to begin a sentence with a conjunction. And before the flames begin, just let me go grab my Fowler, Chicago and Garner so I can quote page numbers :-).