How is an identity error message for enumerating a sleep validator?

Question

I have a Spring MVC web application that uses JSR303 validation:

import javax.validation.constraints.NotNull;
import org.hibernate.validator.constraints.NotBlank;

class User {
    @NotBlank(message = "user name is mandatory")
    String userName;

    public enum Color {RED, GREEN, YELLO}

    @NotNull(message = "color is mandatory)
    private Color color;
}

When my web controller validates the user, it reports that "color is required" if this parameter is not specified. This message is displayed in a web form. However, if the string "BLUE" is passed in color (which is not one of the 3 enumeration options), I get the message as follows:

Failed to convert property value of type java.lang.String to required type User$Color for property color; nested exception is org.springframework.core.convert.ConversionFailedException: Failed to convert from type java.lang.String to type @javax.validation.constraints.NotNull User$Color for value BLUE; nested exception is java.lang.IllegalArgumentException: No enum constant User.Color.BLUE.

This message is displayed in a web form. How to personalize the message?