The maximum prescribed height for a depressible stud is 25 mm and for a non-depressible stud 20 mm (regulation 32). COLOUR OF ROAD STUDS . 6.9 Retroreflecting road studs are prescribed for use with reflectors coloured white, red, amber or green (regulation 31 (7)). The colours are specified by chromaticity co-ordinates in BS EN 1463-1 and are.