Bug 152430 Key can modify to empty string
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/CustomColorHandle.java
* color name to identify a custom color.</dd>
 * <dt><strong>color </strong></dt>
 * <dd>The color value of the color, it is preserved as the user typed in.
 * </dd>
 * <dt><strong>Resource Key </strong></dt>
 * <dd>a custom color has an optional display name ID to localize the display
 * name.
 * </dl>
 * 
 * <p>
 * The color value can contain any of the valid color representations:
 * <ul>
 * <li>INT_FORMAT: An integer with a valid RGB color value.</li>
 * <li>HTML_FORMAT: ( #RRGGBB or #RGB )</li>
 * <li>JAVA_FORMAT: ( 0xRRGGBB )</li>
 * <li>CSS_ABSOLUTE_FORMAT: ( RGB(r,g,b) )</li>
 * <li>CSS_RELATIVE_FORMAT: ( RGB(r%,g%,b%) )</li>
 * </ul>
 * <p>
 * If the color value is in one of the format, then it is meaningful to retrieve
 * it as integer RGB value. Otherwise the color value is treat just as what the
 * user has entered, and can not be converted into an RGB value.
 * <p>
 * Use {@link ColorUtil#format(int, int)}and
 * {@link ColorUtil#format(String, int)}to do the formatting work on a color
 * value.
 * 
 * @see ColorUtil
 */
public class CustomColorHandle extends StructureHandle