In Flask, how can I write a condition in a file py (not html) and print only result in html?

I'm new to Flask. In the index.html page, there are a combobox and a textbox. If I select the Red item of the combobox, I would like the textbox to print "You have selected the item Red".

I would like to achieve this inside in the python file and not in the html file. I saw that conditions can also be written in the html file, but I don't want to write them there. So I would like to write the condition in the app.py python file and NOT in the index.html. Something like this: if colours.get() == "Red". Obviously, however, important, i would like to display the output in the textbox index.html

How can i get this? Thank you all!

Flask Code: app.py

from flask import Flask, render_template

app = Flask(__name__)

@app.route('/')
def index():
    return render_template('index.html')

@app.route("/combobox", methods=["GET","POST"])
def dropdown():
    colours = ["Red", "Blue", "Black", "Orange"]
    return render_template("index.html", colours=colours)

if __name__ == '__main__':
     app.run(host="0.0.0.0", port=8080)

Textbox and combobox code: index.html

  <form name="Item_1" action="/getLigand" method="POST">
    <select name="colours">
      {% for colour in colours %}
     <option value="{{ colour }}" SELECTED>{{ colour }}</option>
      {% endfor %}     
   </select>
  </form>
  
<form action = "/result" method = "POST">
  <input type="text" name="result" />
  </form>

Here is an example that works. This is app.py:

from flask import Flask, render_template, request

app = Flask(__name__)

colours = ["Red", "Blue", "Black", "Orange"]

@app.route('/')
def index():
    return render_template("index.html", colours=colours, chosen="Black")

@app.route("/combobox", methods=["GET","POST"])
def dropdown():
    picked = request.form['colours']
    if picked == 'Red':
        message = "<<< You chose red"
    elif picked == 'Blue':
        message = "<<< You chose blue"
    elif picked == 'Black':
        message = "<<< You chose black"
    else:
        message = "<<< Oh no, you chose orange"

    return render_template("index.html", colours=colours, chosen=picked, message=message)

if __name__ == '__main__':
     app.run(host="127.0.0.1", port=8080)

This is templates/index.html:

<form name="Item_1" action="/combobox" method="POST">
    <select name="colours">
      {% for colour in colours %}
        {% if colour == chosen %}
         <option value="{{ colour }}" SELECTED>{{ colour }}</option>
        {% else %}
         <option value="{{ colour }}">{{ colour }}</option>
        {% endif %}     
      {% endfor %}     
   </select>
   <input type="submit">
</form>
  
<form action = "/result" method = "POST">
  <input type="text" name="result" value="{{ message }}" />
</form>

Followup

Now, having written all that, I'm starting to wonder if you wanted that message to show up as soon as they PICK the color, without having to click a "Submit" button. If that's what you're after, that can be done, but it's a very different technique. That all has to be done in Javascript, as code that you embed within your HTML. That's all handled entirely within the browser. It doesn't send anything to the web server, so your Python code will not be involved.