A reaction occurs in which dichromate ions convert to chromate......
Reacting potassium dichromate with potassium hydroxide causes a reaction in which potassium dichromate is converted to potassium chromate.
Dichromate and chromate ions are different colours (orange and yellow respectively) due to different arrangement of electrons in the 3d-sublevel of the chromium.
The d-orbitals are degenerate but when water molecules form a complex with chromium ions, the energy levels split. The difference between the energy levels of the split d orbitals relates to the wavelengths of light the ion absorbs, and it does that by electrons being moved from one split level to the other. The wavelengths that are not absorbed are detected by the human eye. In the case of dichromate the wavelengths not absorbed correspond to the orange region of the visible spectrum, and in the case of chromate, yellow.