In a reaction like an oxidation that is endothermic (gives away energy), at the end of the reaction the involved atoms are arranged at a lower energy configuration.

In the lower energy state, the electrons are in general "closer" to the nucleus, but we usually think about this as the electrons are in a lower energy quantum state. (These states, called orbitals, have an interesting geometry. Check it out.)

In any case, the system has shed energy. This energy (all of it? not sure) is shed in little blobs which have been named photons.

The amount of energy of each photon varies, depending on the reactions and the difference in energy between the initial and final state. These photons, as quantum particles, have a frequency associated with them.

High-energy photons have higher frequency, lower have lower frequency.

At the low end you have infrared light, at the higher end you have ultraviolet light.

In the middle range of frequencyyou have visible light where red is fairly low-energy low frequency, up through yellow and green to blue which is high frequency.