Well, I wouldn't say it is really only colloquial, just depends on what you emphasize in a sentence.
If you start a conversation with it then yes, it's colloqiuial, like calling someone up on the phone and asking Агов, ти де? (Hey, where are you?)
But it could be like AngryPeasant said below: -Я в кіно. - Ти де??? (- I'm at the cinema. - You're where???)
Yes, it's like "ty". There is no sound for "и" in English really, so both "i" and "и" may sound like "i" to you. The difference is that "и" always leaves the preceding consonant "hard", whereas "i" makes it "soft" (palatalized). This way "ни" would be pronounced close to "ni", if you read it in Spanish, and "ні" - like "ñi". But people will still understand you, even if you always pronounce it as "i", so don't worry too much, it will come with time ;)