I'm sure there was an official colour for the RAF training scheme, but I suspect the exact specification was sometimes down to the supplier. I base my comments on flying RAF training aircraft for about 8 years as a student and instructor. The Jet Provosts and Chipmunks I flew were quite a dark red. But later, when instructing on Chipmunks, the ones coming back from major servicing seemed to have been painted in a more vivid red with a slight hint of orange. The Tucanos supplied from Shorts were a different shade altogether with a brighter colour than the JPs.
There are a lot of factors - for example paint technology and regulations as to what goes into paint can change. Also, red pigment is less UV stable than others so the colour will change over time - the JPs longest out of major serviving had noticeable fade. Plus you need to realise any colour image of aircraft won't necessarily give an accurate representaiton due to film differences and/or digital reporduction.