In a medium bowl combine flour, cream of tartar, baking soda and salt. Set aside. In a large bowl or stand mixer cream butter, shortening and sugar until light and creamy. Scrape bowl as needed and prior to each new addition. Add eggs and zest. Mix until just barely combined. Slowly begin adding flour mixture. Mix on med/low until fully combined. Chill dough for at least 15 minutes.

Meanwhile prepare desired sanding sugar into small bowls.

Grabbing a teaspoon amount of dough, roll into neat balls. Gently roll dough ball into desired sanding sugar color. transfer coated ball to a prepared baking sheet. Repeat with remaining dough, spacing balls at least 2 inches apart (minimum of 12 will fit a regular baking sheet).

Bake for 7-8 minutes or until edges are just slightly turning a golden color. Let cool on pan for 1-2 minutes before removing and transfering to a wire rack to cool completely.

Store in an airtight container for up to several days. Any remaining dough can be frozen for up to several months if sealed well. Label properly with name and any important instructions such as bake temp and time.