On 1.1.2014 at least 24 pot shops opened in Colorado and I wonder if smoking marijuana is now legal or just tolerated as long as you don't smoke on the street (like in Amsterdam). What are the differences between Colorado and Amsterdam? Are there any restrictions for tourists?
It's 100% legal now, within some limits. Wikipedia summarizes the situation neatly:
Adults aged 21 or older can grow up to three immature and three mature cannabis plants privately in a locked space, legally possess all cannabis from the plants they grow (as long as it stays where it was grown), legally possess up to one ounce of cannabis while traveling, and give as a gift up to one ounce to other citizens 21 years of age or older. Consumption is permitted in a manner similar to alcohol, with equivalent offenses proscribed for driving. Public consumption remains illegal.
The one big difference to Amsterdam is that, unlike Dam's coffeeshops, you are not allowed to smoke pot in the stores that sell it, or any other business open to the public. In practice, this means the only place you can fully legally smoke is at your home (or someone else's with their permission).
This article has more practical details, including the important note that, without Colorado ID, you can only buy 1/4 oz (~7g) from a shop. Bringing cannabis to the airport is also illegal, and taking it out of state lines is also a bad idea (because it's illegal everywhere else except, soon, Washington).