Use an Event Dispatcher. Set up a new one in your secondary actor called OnColorChanged and give it an input of Linear Color. During BeginPlay in your primary actor you bind a custom event ChangeColor to the event dispatcher of your secondary actor. If you now call the dispatcher after you changed the color of your secondary actor in your input event the primary actor will receive a call to ChangeColor with the new color.
I don’t know what you want to achieve but this sounds like a problem that could be solved more easily with a Parameter Collection. This way your colors are always synchronized and you won’t need to worry about events and so on.