SU is classic red, but Mazda also did a classic red in A3E (also referred to as Eternal Red) There are also other reds using codes D7, SQ, RH, 15Y, 18U too... The list goes on and on... there's a lot of reds, but as far as I know, no others called 'Classic Red'
It's also worth noting that sometimes the same colour has different names for different regions. For example the metallic blue cars paint is Pacific Blue or Caribbean Blue 4Y depending which continent (Europe/Japan or America) and the Classic/Eternal Red A3E.
I've only ever seen 1 kind of red on these cars and they have all looked pretty much the same, that's not to say there isn't a subtle difference in shades.
It might be cheaper to have the boot lid 'mopped' to restore the colour... of if it's the bodywork at fault, then get that done instead.
You could end up painting the bootlid and still not having a match... which would waste about £150-200.. For about £50-75 you should be able to get the car mopped professionally and see if that does the trick.
There's also the possibility that the car has been repainted once already.
Paint codes
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