http://www.designinsite.dk/htmsider/m1317.htmmaterials you'll need to build the straw ...

I don't think that some thermochromic materials are entirely safe for food contact, however as the website shows, having them incorporated into the container would probably do.

The idea of a portable, cheap (and disposable) add-on to disposable coffee cups means well but I don't think resolves much regarding the issue of people burning themselves with hot liquids. I think the issue has been solved by McDonalds by just printing the words "Contents may be hot". That covers your lawsuit. A more expensive stirrer with a changing color code would not stand out against the cheaper stirrers with only one color. It would have to be big enough to be read and understood as a semantic object, and then thrown away. It would be a nice temporary gimmick though.

Funny, this idea struck me independently recently and I saw no one was doing it so I patented it and am producing a very similar item to what you've described. I like the idea of a three color stirrer and I love the name but, unfortunately I didn't come up with this name so I have no right to take it. The name I chose was "Safety Stirrer". I decided to create it for the exact same reasons you hit on- corporate liability and consumer protection. I personally don't like having my tongue unexpectedly burned and I see the prevention of that as a great benefit.

As for the three color idea, it's ideal but it would take complex manufacturing methods that would put such an item out of financial reach for most businesses. Consider, businesses are giving these away and eating the cost. What I made is a two color- green = safe, white = hot. A problem you didn't notice is 'just right'. 'Just right' varies by a person's pain threshold. What I sought and created was a balance between hot and safe using scald data. That temp turns out to be 140F/60C, which is actually a very comfortable drinking temp in my opinion.

As for the comment that thermochromic dyes pose health risks- simply not true at all. Also, one has to consider what conditions one might ingest the dye. In the case of a plastic object impregnated with the dye (i.e. evenly mixed prior to shaping and cooling) the only way to injest is to actually eat the stirrer. It doesn't flake off because it's not a coating.

If anyone cares to check them out my site is "Safetystirrer.com". Hope one of these meets you at your local coffee shop soon and saves a tongue or two.

Fourthfield, another sales approach might be to offer small packets of them at stores for a low price, maybe 99 cents for a dozen or so. I'm assuming they could be reused a number of times. If they're not reusable, you should sell larger packs. People could buy them for use at home, in their cars, or at work. I wouldn't mind having a few next to my microwave. Many people have coffee brewers or microwave ovens at their businesses. Good luck.