I went through this and even sent my negatives to Kodak to see if there was a problem.
Conclusion, my scanner sucks, and the best way to get proper colors from Ektar is to have them PRINTED or scanned by an actual professional scanner. And NO Vuescan won't cut it.
Kodak sent me back an optical print that they made from my negative that had scanned like crap and it had outstanding color.
Sorry to say this, but the best way to get great prints from Ektar is to pay someone to provide you with prints, or make them yourself.