Currently this is not possible in DxDesigner, the symbol's attributes such as colour and line style are not displayed in the properties dialog. If you consider this a useful feature then post an enhancement request over on the Ideas site.
You means that even automation can't support to color symbol instances?
Today, more and more young designers prefer to coloring symbol instances during entrying. I think they may hope the coloring can help them to take care, or as a reminder to take modification ,change or corretion late. It's important to our young desingers. I really hope your team consider to support this feature in 7.9.4.