Hi Niel,
I've coppied this from one of the posts else where on this site.
Here in the U.K., Mk Polychromatic Blue has the I.C.I. ref PO31/4863 and is made up as follows:
Clear Base 404 ------- 488 parts
PO30/9995 ------- 170 parts
PO30/9970 ------- 20 parts
PO62/9970 ------- 114 parts
PO63/9904 ------- 82 parts
PO62/9900 ------- 64 parts
PO62/9949 ------- 38 parts
Even if this brand of paint is not available in your part of the world, your local supplier should be able to cross-reference it, and provide a good match.
Regards
Martin