To answer this, we spoke to Joe Mason at Reliant Spares. The 'yellow top' engine was fitted in the Rialto 2 in 1984 and came with a very aggressive head skim, a different camshaft with wider lobes, different bob weights in the distributor and a different needle in carb - also the drive on distributor base is turned 190-degrees.

Red rocker covers tend to be 1970s Robins/Kittens, blue is normally reconditioned or if from a Reliant Rebel was a blueprinted engine from factory - they're very sweet runners, a green top was an 'unleaded conversion' at factory (debatable as all Reliant engines have hardened valve seats and are designed on run on cheap fuel) set up for advanced timing to stop 'pinking'.