It can't be settable elsewhere. What if you tried ...

# tornadofx

[Ruckus]

It can't be settable elsewhere. What if you tried this in a style?

redIcons {
    iconColor.value = c("red")
}
greenIcons {
    iconColor.value = c("green")
}

The value is just forwarded on to the current internal property holder (there's one created for each selector). Otherwise the property could only be set to a single global value, and would be useless.