Thanks for reaching out. Unfortunately, what you are trying to do is not currently supported in Lucidchart. However, we're always looking for ways to improve Lucidchart, and your feedback is the best way for us to do that. Would you mind adding your idea to our feature request form? Ideas submitted to this form are passed on to the product development team for their consideration.
If your layer visibility is being toggled with an action, you could use the action to toggle the visibility of a duplicate shape layer with the color you want. Currently though there is no way to do this with conditional formatting.
Thanks for your request and I apologize for any inconvenience this causes you.
For anyone else looking for information on conditional formatting, see this article: Conditional Formatting and Icon Sets