Highly colored thin films were prepared by irradiating transparent resins consisting of the iodonium salt [4-[(octyloxy)phenyl]phenyl]iodonium hexafluoroantimonate (OPPI, 1) and a dye precursor such as crystal violet lactone (CVL, 2) dissolved in an acrylate epoxide mixture. It has been demonstrated that a mixture of CVL and OPPI is a very efficient photoinitiator for radical polymerizations. Color formation takes place via the excited state of CVL, that is oxidized by OPPI to a radical cation, that subsequently forms the colored triarylmethane cation by hydrogen abstraction from the solvent. Formation of photoacid via the excited state of OPPI also contributes to color formation. The main advantage of our approach, by which films of different colors can be made, is that highly colored (OD > 2.0 : 400-700 nm; 15 mu m thick) films can be formed at rates comparable with those of noncolored films.