which colour show maximum Photoelectric effect IN rainbow

Answer 1

It's a little unclear by what is meant by "maximum effect". If by that you mean "for which colour (wavelength) will electrons have the highest energy", then we need just look at the equations Kmax = hν - φ and c=λν (where Kmax is the maximum kinetic energy of the electrons, h is Planck's constant, ν is frequency, φ is the work function, c is the speed of light in a vacuum, and λ is wavelength). Electrons will have the highest energy when the incident light has the highest frequency, which according to the second equation is the one with the shortest wavelength. Since we perceive colour as wavelength, the answer is the colour with the shortest wavelength - violet.

If something else is meant like efficiency, we need to consider the metal involved and its work function (minimum energy required).