Question

Why are cathode rays green in colour ?

Answer 1

There is gas inside the Cathode ray Tube that emits electrons after getting ionised.
High voltage is applied to knock electrons from the gases. In a short period of time, seconds or a fraction of a second, these electrons rejoin the ionised molecules, fall back into the ground states and emit energy in the form of photons in the process that depends upon the nature of the gas. The gas used is generally oxygen and hence the colour of the gas is mostly green.