I'm not a paint expert, but if its not the Rover standard colour (eg Targa Red) as indicated by the code (my Targa code is "CNE") on the identity plate on the O/S door pillar, which can be tied back to the Rover range from Halfords, Holts, CarPlan, etc, then you could try the following.
Remove your Petrol filler cover plate and take it to you local paint factor (eg Nuagane, or similar from Yellow pages) and ask them to mix up a matching colour and put it in an aerosol for you. They can do a spectrum analysis to get the correct pigments.
If you have a half litre or litre made up you apparantly get a better mixing match and you could have a couple of aerosols made up and keep the rest for other means of application. I know it's not the cheapest option but you might get a close match if the car has been repainted a non std colour and at least your touch-ups would all match in the future.
The other consideration is what type of paint to get. I believe Cellulose and Acrylic would be available in aerosols but perhap only Cellulose and TwoPack from a Paint supplier.
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Ramon