diff --git a/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/UIDefaultsLoaderAccessor.java b/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/UIDefaultsLoaderAccessor.java
new file mode 100644
index 00000000..3c5c8b5b
--- /dev/null
+++ b/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/UIDefaultsLoaderAccessor.java
@@ -0,0 +1,29 @@
+/*
+ * Copyright 2020 FormDev Software GmbH
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.formdev.flatlaf;
+
+/**
+ * Enable accessing package private methods of {@link UIDefaultsLoader}.
+ *
+ * @author Karl Tauber
+ */
+public class UIDefaultsLoaderAccessor
+{
+	public static int parseColorRGBA( String value ) {
+		return UIDefaultsLoader.parseColorRGBA( value );
+	}
+}
diff --git a/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeEditorPane.java b/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeEditorPane.java
index 4500c18e..27de8f8d 100644
--- a/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeEditorPane.java
+++ b/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeEditorPane.java
@@ -54,6 +54,7 @@ class FlatThemeEditorPane
 		textArea = new FlatSyntaxTextArea();
 		textArea.setSyntaxEditingStyle( FLATLAF_STYLE );
 		textArea.setMarkOccurrences( true );
+		textArea.addParser( new FlatThemeParser() );
 
 		// theme
 		try {
diff --git a/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeParser.java b/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeParser.java
new file mode 100644
index 00000000..96f80d95
--- /dev/null
+++ b/flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeParser.java
@@ -0,0 +1,64 @@
+/*
+ * Copyright 2020 FormDev Software GmbH
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.formdev.flatlaf.themeeditor;
+
+import javax.swing.text.Element;
+import org.fife.ui.rsyntaxtextarea.RSyntaxDocument;
+import org.fife.ui.rsyntaxtextarea.Token;
+import org.fife.ui.rsyntaxtextarea.parser.AbstractParser;
+import org.fife.ui.rsyntaxtextarea.parser.DefaultParseResult;
+import org.fife.ui.rsyntaxtextarea.parser.DefaultParserNotice;
+import org.fife.ui.rsyntaxtextarea.parser.ParseResult;
+import com.formdev.flatlaf.UIDefaultsLoaderAccessor;
+
+/**
+ * Parser for FlatLaf properties files that checks for invalid values.
+ *
+ * @author Karl Tauber
+ */
+class FlatThemeParser
+	extends AbstractParser
+{
+	private final DefaultParseResult result;
+
+	FlatThemeParser() {
+		result = new DefaultParseResult( this );
+	}
+
+	@Override
+	public ParseResult parse( RSyntaxDocument doc, String style ) {
+		Element root = doc.getDefaultRootElement();
+
+		result.clearNotices();
+		result.setParsedLines( 0, root.getElementCount() - 1 );
+
+		for( Token token : doc ) {
+			if( token.getType() == FlatThemeTokenMaker.TOKEN_COLOR ) {
+				try {
+					UIDefaultsLoaderAccessor.parseColorRGBA( token.getLexeme() );
+				} catch( IllegalArgumentException ex ) {
+					result.addNotice( new DefaultParserNotice( this,
+						"Invalid color.\n\nUse #RRGGBB, #RRGGBBAA, #RGB or #RGBA",
+						root.getElementIndex( token.getOffset() ),
+						token.getOffset(), token.length() ) );
+				}
+			}
+		}
+
+		return result;
+	}
+}
diff --git a/flatlaf-theme-editor/theme-editor-test.properties b/flatlaf-theme-editor/theme-editor-test.properties
index 18bf541b..4fe494bb 100644
--- a/flatlaf-theme-editor/theme-editor-test.properties
+++ b/flatlaf-theme-editor/theme-editor-test.properties
@@ -15,6 +15,8 @@ Prop.color1=#f80
 Prop.color2=#ff8800
 Prop.color3=#f804
 Prop.color4=#ff880044
+Prop.color5=#12
+Prop.color6=#12345
 Prop.border=1,2,3,4
 
 Prop.scaledInteger={scaledInteger}123