I want to select a certain color using my camera so only that will be recorded as color, with all the rest in gray- or b/w-scale. Meaning, only one color and its shades appear colored, and rest is b/w. Something like the picture below:
http://blaberize.com/wp-content/uploads/2009/12/selective-color-wallpaper-city.png
My answer is going to be short, on this one:
Why? Because you might wish later that you had the full-color image down the road. Secondly, post-production will almost always give you a better result than the camera itself will. There are many options that will do this in Photoshop, Paintshop pro, Gimp, etc.
This is not done in-camera but in post processing. If that structure was the only red thing in a frame then selective desaturation would have done the trick, else it is pretty much just selecting the structure and desaturating the rest.
I would take a normal picture and later process it in some software. There is Tintii for example where the standalone version is free, so you can try it. Here is a nice example taken from the Tintii photo stream on Flickr:
Then there is a "manual" approach possible - in GIMP for example. I used this technique to create this picture.
The answers here so far are spot-on, but in the interest of those who stumble on this question at some later date, I'll point out that there are cameras that are now able to do selective coloring in-camera (ex: Nikon D5100).
In the case of still photography, you still really want to do processing after the fact because you'll be able to do a better job, and because you'll (hopefully) keep a good original that you can use if you ever change your mind. If you happen to want to do do selective coloring in a video capture, however, the in-camera option becomes significantly more attractive.
Several Canon Point and Shoot models do allow this, and its known as Color Accent. They also usually offer Color Swap, which lets you change the color of an object in a photo.
The crazy good Canon S95, SX130 and nearly all SD models do this as well, though I do not think the AS models have this feature.
As you have found, the DSLRs from Canon, including your T1i, do not support this feature.
One downside to using this in camera is that you will only get this image: there will be no 'original' to use in other ways. You will only get this color accented image. This is the downside to doing in-camera editing or using in camera settings. (for the S95, if you shoot RAW+JPG, however, the RAW will be untouched, the JPG could be color accented.
Some cameras have this feature as a photo effect. Or you can use photoshop to selectively desaturated (remove colours ) parts of the image ( desaturated those parts which you want to make black and white )
