Why is it not possible to construct arguments to methods using Strings? [duplicate]-Collection of common programming errors

My idea was to use casting to get a StringBuilder object to pass a valid argument to a method. Like this:

public TTTButton(String color) {
    StringBuilder sb = new StringBuilder();
    sb.append("Color."+ color.toString().toUpperCase());
    this.setBackground((Color)(Object)sb);// Runtime Exception: java.lang.StringBuilder cannot be cast to java.awt.Color 

Please share your thoughts on why such things are impossible (or maybe they are possible)?