Question

what changes in colour of iron nails and copper sulphate solution do you observe after keeping the iron nails dipped in copper sulphate for about 30 minutes

Answer 1

If this is an experiment you were supposed to do, write down what you observed.

Had you done the experiment, you might have gotten a result similar to
http://library.thinkquest.org/18406/sdlab.htm

Answer 2

When iron nail is immersed in copper sulphate solution, there is change in color composition. Blue color changes into green color. This happens because iron is more reactive than copper, so change in color happens.