Toasters with a toast color sensor has been around for at least a couple of years if not longer. They just don't advertise them that way.

I remember seeing a guy design a toaster that had a component from a smoke detector in it which (i'm quoting form memory here) monitored the amount of carbon or something being released (???).

The problem, as I'm sure we're all aware, is that toasters are very slow when you start using them, but if you make three or four rounds of toast one ofter the other, each batch toasts quicker. This smoke detector method solved that problem....

Although I've not seen it on the market, so perhaps it didn't work. But still, a novel idea.

There are already "transparent" toasters with windows on their sides. This is low-tech, but I imagine works better and is cheaper than having a camera inside your toaster (an environment which is dark and hot).

This has sort of existed since the '40s, using radiant heat from the bread: http://www.automaticbeyondbelief.org/

I haven't tested this, but you're probably have issues with e.g. dark rye.

What if you eat rye bread?