well.. I just got my 90 mustang repainted with the stock red which is Ford color code EP. There should be a sticker on the back of the drivers door when you open it with vin number and a bunch of other codes including the paint code. Also look at http://www.mustanggt.org , they have all the info for every year mustangs.
there are a few different reds for that year acctually.. i just checked gt registry and this is the codes of the reds.
EP = Bright Red
EH = meduim cabernet red
ELYJ = wild strawberry metallic clearcoat/titanium lower