In order to change the color your parts in the Optimizer, you can go into the Parts tab of the Product Editor in Mozaik. Click Edit, and select the preferred color from the drop down:
Once you send the parts to the optimizer, you will then see the color both in the Parts tab of the Optimizer as well as the Patterns tab:
Parts tab:
Patterns tab. Note that the flip side operations color will override the chosen color in the Patterns tab: