About Snape's true colors, I think Dumbledore was right in withholding that information, because had Harry known the truth about Snape since starting Hogwarts, he would have formed a very bad opinion about his father, and many of his actions would have been ruled by his emotions. The story would have been totally different, because many things happened due
to Harry's strong dislike for Snape. Also, he may have tried to get closer to Snape feeling sympathy for him, and Snape would not have liked that, because as much as Harry had his mother's eyes, he was still a carbon copy of James. Snape did not want to be reminded of his past, and what "could have been" had Lily lived. Snape also requested Dumbledore not to tell anyone about his love for Lily, and Dumbledore respected that wish.
One more thing: Even though Lily was Harry's mother, Snape's love for Lily was totally between him and Lily, and since Lily was dead, it was his secret, and truthfully, none of Harry's business. So I don't grudge Dumbledore for withholding that info.
About the Prophesy though, I believe Dumbledore withheld that too long. I agree with him that in Year 1 and Year 2, telling Harry about the prophesy would have been too much to bear for a 11 or 12 year old (especially one who found out about magic at the age of 11). I also don't think his 3rd year was a good time either, because Harry was overwhelmed with finding someone "family-like", and thrusting the prophesy on him would have been too much. But 4th year, after Harry saw Voldemort resurrected, was the perfect time to tell Harry about the prophesy, because though Harry was still coming to terms with seeing Voldemort return, it would have give him time to think about everything and plan accordingly had he known about the prophesy. And also, he would have tried much harder at Occlumency during his 5th year if he knew of the real consequences of Voldemort invading his mind, and he might have given it a thought that Sirius may not have been taken prisoner by Voldemort, and even alerted the Order Members to check up on Sirius,
But then, if you think about it this way, telling someone that they had to kill someone else for the future of the Wizarding World is pretty big, and though I don't completely agree with Dumbledore's train of thought, I can see and understand the reasoning behind it.
I don't think Dumbledore had a total right over keeping much of his information about Harry's future, but it ended up helping Harry in the end, because as he went in search of "that information" he learned a lot about the Deathly Hallows, Dumbledore's past, and also about his parents' past. In the end, he got more info than ever because Dumbledore kept much of his info. Follow my train of thought?