As far as anyone knows, there is no US equivalent for Imperial Blue. The paint code is either Q2 or Q3 depending on the year the RS was built, but they both look the same. A number of RSOC guys order paint repair kits from this website in the US (
http://www.drcolorchip.com/) and use Sonic Blue to match their paint. They say it matches perfectly.
Now, I've seen it said a number of times on both FF and FJ that Sonic Blue and Imperial Blue are not the same, but I've seen an Imperial Blue RS wing mounted to a Sonic Blue SVT and it looks the same to me.
I would just use Sonic Blue.