How can I replace the text in JSON string with a userscript for Greasemonkey

I want to create a user script for Greasemonkey in Firefox without using jQuery, which can replace old text by new text when the page of website is loaded.

HTML code:

..

window.app = profileBuilder({
..
    "page": {
        "btn": {
            "eye": "blue",
            "favorite_color": "blue",
            "gender": "male",
        },
    },
..
});

..

Replace "blue" of eye by "green", "blue" of favorite color by "red" and "male" by "female".

When the page will be loaded, I want to see, for instance Green (not Blue) for Eye and Female for Gender (not Male).

I guess I need to use functions next:

GM_getValue()
GM_setValue()
JSON.parse()
JSON.stringify()

PS: the code JSON is directly in the page and not in file (../code.json)

Userscript code:

// ==UserScript==
// @name        nemrod Test
// @namespace   nemrod
// @include     http*://mywebsite.com/*
// @version     1
// ==/UserScript==
var json = {"page": {"btn": {"eye": "blue","favorite_color": "blue","gender": "male",},},};
var stringified = JSON.stringify(json);
stringified = stringified.replace(/"eye": "blue"/gm, '"eye": "green"');
stringified = stringified.replace(/"favorite_color": "blue"/gm, '"favorite_color": "red"');
var jsonObject = JSON.parse(stringified);

It doesn't work

Can somebody help with the right code?