In-package cold plasma processing is highly desirable in the food and biomedical industries as it allows for efficient sterilisation, and prevents against post-packaging contamination. The sensitivity of methylene blue dye to the reactive species generated from cold plasma is tested in this work for possible use as a marker of process efficacy. A large gap dielectric barrier discharge (DBD) operating in air was employed to generate the plasma discharge within the sealed package. The discolouration of methylene blue dye placed inside the package was studied as a function of DBD operation time, applied voltage and spatial position. Ozone concentrations were measured immediately after treatment as an indicator of one of the key meta-stables produced by the approach with values of up to 1800 ppm recorded. Visible absorption spectra and pH changes of the dye were measured. A decrease in peak absorbance of the dyes and pH was observed as a function of treatment time and ozone concentration. Optical emission spectroscopy of the discharge revealed the generation of excited nitrogen and reactive oxygen species. The results of kinetic modelling revealed that the dye discolouration can be used as a suitable marker reaction for treatment times within the order of 30 s.