In studio terms, red is always associated with ‘Record’ so would be confusing if used for ‘stop’. The Sonifex machines followed the convention of the earlier ITC machines which used yellow for ‘ready’ (i.e. the cart was cued and ready to go) and green for ‘start’. That was also the repeater indicator layout on the channel strips of the Type D continuities (used by BBC Network Radio in the 1970s and 80s) and the later Radio 1 ‘Maxicons’.