The easiest ivory tone I know of is, if youre starting with a white material as the base, dust the surface with Mohawk pickle beige. Its rattle can number "M101-0303, Tone Finish Toner Beige Tone/Pickle". They have other colors that may suite your personal end result if the intention is older ivory vs a newer ivory. If you look at the Colt Walker that Michal Madsen uses in the Hateful 8, the grips are molded off originals and cast in high impact plastic then simply dusted with the Mohawk pickle tone. The ivory wouldnt have been that old at the time the film was set in. The only set back you may have with surface finish's is just that, they are only surface deep and prone to damage if handled regularly. For a display prop or even moderate on set use, its not a problem.
But if youre going to cast them, and know the materials very, very well, casting them in color locks the finish in. There is no paint in anything in this photo, everything is literally as is out of the mould in the attached photo.