Move method for detecting project colors to QgsProperty

python/core/auto_generated/qgsproperty.sip.in

@@ -311,6 +311,13 @@ Returns true if preparation was successful.
 %Docstring
 Returns the set of any fields referenced by the property for a specified
 expression context.
+%End
+
+    bool isProjectColor() const;
+%Docstring
+Returns true if the property is set to a linked project color.
+
+.. versionadded:: 3.6
 %End
 
     QVariant value( const QgsExpressionContext &context, const QVariant &defaultValue = QVariant(), bool *ok /Out/ = 0 ) const;

src/core/qgsproperty.cpp

@@ -418,6 +418,13 @@ QSet<QString> QgsProperty::referencedFields( const QgsExpressionContext &context
   return QSet<QString>();
 }
 
+bool QgsProperty::isProjectColor() const
+{
+  QRegularExpression rx( QStringLiteral( "^project_color\\('.*'\\)$" ) );
+  return d->type == QgsProperty::ExpressionBasedProperty && !d->expressionString.isEmpty()
+         && rx.match( d->expressionString ).hasMatch();
+}
+
 QVariant QgsProperty::propertyValue( const QgsExpressionContext &context, const QVariant &defaultValue, bool *ok ) const
 {
   if ( ok )

src/core/qgsproperty.h

@@ -352,6 +352,13 @@ class CORE_EXPORT QgsProperty
      */
     QSet< QString > referencedFields( const QgsExpressionContext &context = QgsExpressionContext() ) const;
 
+    /**
+     * Returns true if the property is set to a linked project color.
+     *
+     * \since QGIS 3.6
+     */
+    bool isProjectColor() const;
+
     /**
      * Calculates the current value of the property, including any transforms which are set for the property
      * \param context QgsExpressionContext to evaluate the property for. The variables and functions contained

src/gui/qgspropertyoverridebutton.cpp

@@ -854,13 +854,7 @@ void QgsPropertyOverrideButton::updateSiblingWidgets( bool state )
 {
   if ( state && mFlags & FlagDisableCheckedWidgetOnlyWhenProjectColorSet )
   {
-    state = false;
+    state = mProperty.isProjectColor();
-    QRegularExpression rx( QStringLiteral( "^project_color\\('.*'\\)$" ) );
-    if ( mProperty.propertyType() == QgsProperty::ExpressionBasedProperty && !mExpressionString.isEmpty()
-         && rx.match( mExpressionString ).hasMatch() )
-    {
-      state = true;
-    }
   }
 
   Q_FOREACH ( const SiblingWidget &sw, mSiblingWidgets )

tests/src/core/testqgsproperty.cpp

@@ -92,6 +92,7 @@ class TestQgsProperty : public QObject
     void collectionStack(); //test for QgsPropertyCollectionStack
     void curveTransform();
     void asVariant();
+    void isProjectColor();
 
   private:
 
@@ -1778,6 +1779,22 @@ void TestQgsProperty::asVariant()
   QCOMPARE( fromVar.field(), QStringLiteral( "field1" ) );
 }
 
+void TestQgsProperty::isProjectColor()
+{
+  QgsProperty p = QgsProperty::fromValue( 3, true );
+  QVERIFY( !p.isProjectColor() );
+  p = QgsProperty::fromField( QStringLiteral( "blah" ), true );
+  QVERIFY( !p.isProjectColor() );
+  p = QgsProperty::fromExpression( QStringLiteral( "1+2" ), true );
+  QVERIFY( !p.isProjectColor() );
+  p = QgsProperty::fromExpression( QStringLiteral( "project_color('mine')" ), true );
+  QVERIFY( p.isProjectColor() );
+  p = QgsProperty::fromExpression( QStringLiteral( "project_color('burnt pineapple Skin 76')" ), true );
+  QVERIFY( p.isProjectColor() );
+  p.setActive( false );
+  QVERIFY( p.isProjectColor() );
+}
+
 void TestQgsProperty::checkCurveResult( const QList<QgsPointXY> &controlPoints, const QVector<double> &x, const QVector<double> &y )
 {
   // build transform