The prisoner who is standing at the end can save all other 99 lives by giving his life.

The answer he should say is the colour of the 99th prisoners cap...
Then the answer for 99th prisoner should be clue for 98th prisoner to find his cap...
For example: 99th prisoner is wearing Black cap(100th tells him the colour of the cap) then if 98 prisoner is wearing white cap then the answer for 99th prisoner should be "I'm not wearing White colour cap". Suppose 98th prisoner is wearing Black cap then the answer from 99th prisoner should be "I'm wearing Black cap" and so on.

If the 100th prisoner was lucky then his answer may be correct so we can 100 prisoner sometimes...