Replace element in a string

We have a String, and we want to convert all the characters if it's inside the HashMap. It will convert the character but it will keep the original character and I want to remove it. I tried doing i+1. Also, is it possible to improve the code, to let it detect if the string contains the character, it will converts on it owns? In other words, can I remove all these if statement and still do the job?

Current result:
Red is Redr Yellow is Yellowy GRedreen is Green and what's RedrGreenb?

Expected result:
Red is Red Yellow is Yellow GRedeen is Green and what's RedGredeenb?

import java.util.HashMap;

public class Test {
    public static void main (String [] args){
        HashMap<Character,String> convert = new HashMap<Character, String>();
        convert.put('r',"Red");
        convert.put('y',"Yellow");
        convert.put('g',"Green");

        String str = "Red is r Yellow is y Green is g and what's rgb?";
        String res = "";
        for (int i = 0; i< str.length();i++){
                if (str.charAt(i) == 'r') {
                    res += convert.get('r');
                   // i++;
                }
                if (str.charAt(i) == 'y') {
                    res += convert.get('y');
                   // i++;
                }
                if (str.charAt(i) == 'g') {
                    res += convert.get('g');
                    //i++;
                } else {
                    res += str.charAt(i);
                }
            }
        System.out.println(res);
    }
}