The frequency increases.
I like to think of colour as due to heat(energy) input. As you heat up a piece of steel, it changes colour from none (but it is hot to the touch = infra-red) to deep red to orange to yellow. The hottest flame colour is blue.
Keeping this in mind, but on a much different scale, if we heated the surface you're standing on, first you'd fidget a bit, then start to hop about, and finally dance frantically trying to get away - all examples of frequency.
So as you add energy, the material's atoms/molecules vibrate at a higher frequency, and give off some of the energy in the form of radiation, starting with infrared and changing to visible light as the energy release, i.e.: vibration, gets greater.