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When you +1 something on Google or Google+, the +1 button becomes red and clicking this red button will remove your +1 in the post.
Why was it designed like this? Changing the button color to red make it more eye-catchy and it doesn't directly say "remove me" at all. Wouldn't -1 be a better choice, like the Like and Unlike button of facebook?
I don't work for Google, so my answer will just be a guess at best.
Let's look at this button in basic terms:
download bmml source – Wireframes created with Balsamiq Mockups
It's necessary to make clear that the state is active. Google does this by changing the colour. With this type of button, you would expect that clicking again would deactivate it.
The other reason is probably more important: Google doesn't want to take up much space here to explain what's going on, or going to happen next.
Imagine the button after clicking it would change to something stating the next action when clicking it again, with no more space used to explain the context:
Hang on, what just happened there? Did I +1 this? -1? What happens when I click on it? Can I go to negative by clicking it repeatedly (and start a flamewar)?
The "+1" doesn't mean that you're "addding one" somewhere, it's kind of a brand, mark, "thumbs up" symbol. So, when you click, you're putting your approval on something that is represented by a "+1" symbol. If you want to undo it, you click again in the "+1" and remove that approval.
Think about the facebook "like" button, it could be a "like" getting stronger blue when you click it, and returning to the original color when you dislike it. It's just a symbol.
