Blue

From Tardis Wiki, the free Doctor Who reference

Petrol was a form of blue named after a certain oil used to energise engines within vehicles including cars and trucks.

The Third Doctor wore a petrol blue velvet blazer with a scarlet trim and a notched collar over a crimson velvet waistcoat with a gold fob watch when facing BOSS and giant maggots, as well as during Jo Grant's wedding to Clifford Jones; (TV: The Green Death) also when confronting Mark V Daleks during their Exxilon Gambit, (TV: Death to the Daleks) when confronting Sokolov in 1970's Siberia. (AUDIO: Kaleidoscope) and during his adventures with an older Jo. (AUDIO: Supernature - The Iron Shore)