Utilizing magnetron sputtering to incorporate metal onto polymer surfaces as a substitute for metal parts in the automobile industry offers an eco-friendly alternative. In this work, decorative titanium oxynitride (TiNxOy) coatings were sputter deposited over polycarbonate and polyamide 12 substrates using a low-temperature pulsed deposition method. In order to improve coating adhesion, all substrate surfaces were pre-treated for 30 seconds by reactive plasma etching. Four different film thicknesses were investigated to achieve various colours. The TiNxOy coatings were quasi-amorphous, and the oxygen content decreased with increasing film thickness. Overall, four distinct colours - green, purple, magenta, and blue - were produced, and all coatings showed excellent adhesion to the polymeric substrates. Colour fastness was assessed using adapted pin-on-disk tests to evaluate the film's applicability as decorative coatings for automotive parts, with all films achieving high ratings for colour fastness. This study provides insights into the deposition of thin films on polymeric substrates, showcasing successful approaches to achieve desired colours while preserving the film's optical properties after color fastness testing.