Why is it easier to extract color in HSV model than in RGB?
Say you have an object that is blue, and you adjust the brightness of a lamp in the room, or you move the object so that it does not get as much light. In RGB, all the three components (red, green, blue) will change but in HSV mostly the V (value) component changes. So to recognize pixels belonging to the object by its color, you only need the H (hue) and S(saturation) components of HSV, in contrast to needing all three components of RGB.

Well actually for almost black pixels the H and S components can be anything due to noise, so you will need to see that V is high enough. An equivalent check needs to be done with RGB too.