A preblended additive composition and process for the preparation of polyester resins useful in the manufacture of plastic containers such as bottles is disclosed. The additive composition includes a cobalt-containing compound that is preblended with a phosphorous compound. The polyester resin, e.g., a PET resin, may be prepared by reacting ethylene glycol with dimethyl terephthalate (DMT) or terephthalic acid (TA) in an esterification reaction. The additive composition may be prepared by quickly dumping the phosphorus compound into the cobalt compound such as cobalt acetate, and the resulting composition is added to the reactor vessel before the polycondensation reaction. On the other hand, the additive composition may be prepared by slowly dumping the phosphorus compound into the cobalt compound over a period of at least 5 minutes. Such preblending substantially eliminates any yellow or blue color, or haze in the product article.