I have been wanting to read Heaven Is For Real for some time and wasn't disappointed at all, even with having heard others' opinions of the book.
A couple years ago, I read 90 Minutes in Heaven (by Don Piper), a book along similar lines. More recently, I was reading Heaven (by Randy Alcorn); more theologically based as it is rooted in Scripture vs personal experience. I believe that the spiritual nature God has given us is reflected in our desire to know more about the place God has prepared for us after we die and leave the only physical place we know.
While there isn't 100% surety on the accuracy of what Colton related to his parents about his time spent in Heaven, I have a few thoughts on the subject.
1. God doesn't "spill the beans" and tell us every detail of Heaven in the Bible so as long as the book is accurate as far as what the Bible does say, whose to say that the extra details Colton gives aren't just his imagination. Arguably, they could be just imagination. However, if he talks about Jesus having a horse with rainbow colors, is it possible that's the only way Colton would know to describe such a horse? I've read reviewers who have referred to it as "Rainbow Brite's" horse, but is that a fair comparison? How does one describe a crystal refracting light and iridescent light giving the appearance of shimmering rainbows to a person who has never seen it before?
2. We're talking about comparing Heaven with earthly terms. It's like God giving John a glimpse of future events in Revelation when all he knew was what existed in the first century AD. How can we even imagine what Heaven truly holds for those who truly love God and have been saved through His Son?
3. I can't bring myself to discount Colton's experience because of his age. When one of our children was 3, I saw him standing in our kitchen doorway with his hand slightly out to the side. He was so quiet that I asked him what he was doing and he simply told me that he was holding an angel's hand like it was an every day experience.
Jesus told his own disciples to let the little children come to Him. I lean towards believing that Colton did go to Heaven and meet Jesus and more than that -- I can't wait to meet my Saviour some day too!