Color selector without session

from flask import Flask, request, render_template
import re
app = Flask(__name__)

@app.route("/", methods=['GET', 'POST'] )
def main():
    color = "FFFFFF"
    new_color = request.form.get('color', '')
    if re.search(r'^[0-9A-F]{6}$', new_color):
        color = new_color

    return render_template('main.html', color = color)

<style>
* {
  background-color: #{{ color }};
}
</style>

<form method="POST">
<input name="color" value="{{ color }}">
<input type="submit" value="Set">
</form>
<p>
<a href="/">home</a>

import color
import pytest

@pytest.fixture()
def web():
    return color.app.test_client()


def test_app(web):
    rv = web.get('/')
    assert rv.status == '200 OK'
    assert 'background-color: #FFFFFF;' in rv.data.decode('utf-8')

# This does not set the color
def test_get(web):
    rv = web.get('/?color=AAAAAA')
    assert rv.status == '200 OK'
    assert 'background-color: #FFFFFF;' in rv.data.decode('utf-8')

def test_post(web):
    rv = web.post('/', data={'color': 'AAAAAA'})
    assert rv.status == '200 OK'
    assert 'background-color: #AAAAAA;' in rv.data.decode('utf-8')

    # It is not persistant
    rv = web.get('/')
    assert rv.status == '200 OK'
    assert 'background-color: #FFFFFF;' in rv.data.decode('utf-8')