Here's a Java program to look up an enum by a string value:

enum Color {
    RED,
    GREEN,
    BLUE
}

public class Main {
    public static void main(String[] args) {
        String colorString = "GREEN";

        Color color = Color.valueOf(colorString);

        System.out.println("Color enum value: " + color);
    }
}

In this program, we first define an enum called Color with three values: RED, GREEN, and BLUE.

We then initialize a String variable colorString with the value "GREEN", which is the name of one of the Color enum values.

To look up the Color enum value corresponding to colorString, we use the static valueOf method of the Color enum, passing in the String value as an argument. This method returns the corresponding Color enum value.

Finally, we print out the value of the Color enum variable.

The output of this program will be:

Color enum value: GREEN

Note that the valueOf method throws an IllegalArgumentException if the specified string is not a valid name of an enum constant. To avoid this exception, you can use the EnumUtils.getEnum method from the Apache Commons Lang library, which returns null if the specified enum constant does not exist.