Cs⁺ ions impart violet colour to Bunsen flame. This is due to the fact that the emitted radiations are of :

Bunsen flame is coloured blue by Cs⁺ ions. This occurs as a result of the high energy radiation that is emitted.
The violet spectrum comes right before the ultra violet zone. It is represented by the high energy.

While red colour follows infra red area. It is represented by the low energy.