Welcome to the forum (bit quiet nowadays though ).
Mirage was one of a range of colours which MG Rover offered via a special ordering scheme called Monogram, which gave buyers of new MG Rover cars access to a range of optional equipment, exterior colours and interior trim which were not available from dealer stock.
There were two shades of Mirage - the original Mirage three-coat (paint code IAS) was available from the start of the scheme in 2002 until January 2003, and only 13 MG ZRs are shown on the factory build records as being built in this shade, so very rare now; at least five have definitely been scrapped, and three currently show as unlicenced (so may or may not still be in existance).
Mirage 2 two-coat (paint code IAU) was available from early 2003 until the demise of MG Rover, and 60 ZRs are shown on the build records as having been built in this shade and with more than a third of them now known to be scrapped, Mirage 2 isn't exactly common either.
Your paint code can be found on the VIN plate which should be within the door shut on the passenger side 'B' pillar.
You can find out some of the factory built information by putting your cars' VIN number into the page on www.mg-rover.info (don't enter the first three letters 'SAR') - this will tell you the off-assembly date and which number of that colour that yours is.