Yes, you can choose to capitalize it or not, but it must be a full sentence on its own, not just a partial sentence. For example:
"Apple is the world's most successful company: the iPhone is a major reason for Apple's success." or,
"Apple is the world's most successful company: The iPhone is a major reason for Apple's success."
However, you cannot capitalize the second phrase in this example:
"Apple is the world's most successful company: the iPhone is a major reason."
(because the second phrase is not a complete sentence)
You should also always capitalize a quoted phrase after a colon, e.g.:
She gave me an ultimatum: "Either he goes or I go!"