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Color horses are made from a different plastic than regular white ones. The textural difference is subtle but noticeable when compared side by side in person.
The ones in my collection are the older mold type with the letter B in a circle on the right rear, and have the larger copyright printing on the base with no hole as seen in this comparison photo. This may not be the case in all instances.
The plastic of the color horses is more translucent - it allows some light through and has a softer more porous texture to allow the color ink pens to sink in. Held up to the light you can see the difference best at the mane where the thinner section allows more light through. The regular white horse does not allow any light through at that point.