How to highlight Kotlin's label expression in Idea plugin

I'm trying to write a plug-in to highlight Kotlin's label expression, but it does not work:

class KotlinLabelHighlightAnnotator : Annotator {
    override fun annotate(element: PsiElement, holder: AnnotationHolder) {
        if (element is KtLabelReferenceExpression) {
            holder.createInfoAnnotation(element, null)
                    .enforcedTextAttributes = TextAttributes(Color.BLUE, null, null, null, Font.PLAIN)
        }
    }
}

plugin.xml:

<depends optional="true" config-file="kotlin.xml">org.jetbrains.kotlin</depends>

kotlin.xml:

<idea-plugin>
<extensions defaultExtensionNs="com.intellij">
 <annotator language="kotlin" implementationClass="myplugin.KotlinLabelHighlightAnnotator"/>
</extensions>
</idea-plugin>

Please help me, Thank!