How will i calibrate my qtr array to detect different colours ??
Consider using a color sensor instead. Two possibilities:
Hello, Nouman_Siddique.
The QTR sensors give users a sense of how reflective something is to infrared light. The color of the object influences this result in some way, but so do other factors like surface texture and what material the object is made of. There is also the possibility of external sources of infrared light in your setup that could alter the readings. So, it is not trivial to precisely and accurately detect different colors with the QTR sensors, and it is going to be much more reasonable and straightforward to use hardware specifically designed to sense color, like the two devices Jim linked to.
-Jon