Sorry for a late reply as I was out of City. There are different brands of chinese backview cameras available in market ranging in prices from Rs 1500 to 4000. I chose "Carshibao" brand as it looked robust and had sharper image with good colour reproduction and it costed me Rs 3500/-and came with all the wiring etc.
The coloured lines help u determine how far u r from the object when reversing, whereas the lines on the sides define the width of yr car. Typically if the object is outside the width of yr car , u r safe. but unfortunatly the model available in that shop was probably for less wide cars like alto or swift. Since in my case if u stand behind the car such that u r at the edge of the side line, u will be about 6-9 inches inside the physical width. That means I will have to consider extra margin outside those lines to be safe.
Therefore my suggestion is that select the model in which reference lines are as per width of yr car. the shopkeeper may not understand it so u may have to check it out before actually fixing the camera.