How to isolate a single color in an image
Use the HSV colorspace.  Select pixels that have an H value in the range that you consider to contain "red," and an S value large enough that you do not consider it to be neutral, maroon, brown, or pink.  You might also need to throw out pixels with low V's.  The H dimension is a circle, and red is right where the circle is split, so your H range will be in two parts, one near 255, the other near 0.
I'm using the python OpenCV bindings and at the moment I try to isolate a colorrange. That means I want to filter out everything that is not reddish. 
I tried to take only the red color channel but this includes the white spaces in the Image too. 
What is a good way to do that?