Have you thought about buying two wigs and combining them to create the two-tone effect? It would also make the wig fluffier. Arda and EpicCosplay both have a nice variety of yellow and yellow-orange shades to choose from, and are reliable high-quality sellers. Or, if the cost is an issue, you could get white wefts to add in at the bottom of the base wig.
Coloring a white wig would work, but combining or wefting would save you the trouble of making the dye the right color.