The circle line of the London Underground is actually coloured yellow on the Tube map. You can view the map here: https://tfl.gov.uk/maps/track/tube
This is in contrast to the Central Line which is coloured red on the Tube map. The Circle Line is yellow on both the schematic Tube map and the geographic Tube map.
That said, depending on the colours used to print the map, the yellow of the Circle Line can sometimes appear to be much more of an orange colour.
