There are more to write about the rainbow. Not just about the thoughts, the colors, and so on. Put more life in it. Maybe before you write this, you might want to think of what You were thinking about the rainbow.
Memories, childhood, love and loss, self-reflection, feelings and emotions...
Sometimes, it is not the readers who should keep the thought, but the writer as well should know how to think before pouring out the ink on paper. Capturing the readers' heart is right, but never to forget your own.
I disagree with Venia. I think the rhyme scheme of aabba is fine. However, I do find the rhymes themselves a little trite. Gray/day, far/are...I've heard them a thousand times, and they've become very weak.
This poem could benefit from a little more punctuation, in my opinion. Just to help with the flow.
In the second stanza, I wouldn't pluralize the colors. It throws off the rhyme scheme a bit. Head/reds is a bit of a strain, because it relies on slant rhyme rather than full rhyme.
I think my only criticism for this write is the rhyming scheme. It's a fairly short poem, so I think you should keep the rhyming scheme consistent throughout the whole thing. It makes it easier on the reader, because it just flows better. You can spend less time getting the feel for the rhyme, and more time actually understanding the poem.
But I love your concept. I think it has a great message, and overall it was written beautifully.