Follow the instructions. Generally the instructions on the OMR sheet read as follows:
Use Blue or Black Ball Point pen.
Use Blue pen only.
Use black ballpoint pen Only.
When it comes to using pen, we have a choice of colors: Blue/Black/Green and Red are the commonly available colors. Even sketch pens or markers may be used. But what color of pen should be used?
Since the Optical mark reading process works on the basis of contrast, thus obviously the darker and distinct the mark the better it will be detected. Blue and black colors are more contrast than red or green colors.
Else wise Red color and green colors have their own significance in the academic world. In general the red color is meant to be used by the examiner or teachers to check the sheets or write any comments. Green color is generally meant for the superior, exam supervisor or the principal. So because of this one reason also students are expected to use blue or black pen only.
But more because of the technical reason that blue and black colors are able to create more contrast on a sheet of white paper so they are used. Technically, when a red color mark is scanned and converted to absolute black & white for reading it converts to gray instead of black. Thus the intensity of mark obtained is not perfect, to be detected.
And why not to use any other color sketch pen or marker is discussed more in the next topic.