Lahori1990
Take it to an automative paint shop they will match the colour, and give you hardener also, I recently did paint dashboard and interior panels of khyber, paint shop owner checked the colour mixed three colours gave me enough quantity of hardener (used to paint plastic parts) all things cost me 150 Rs for paint 50 Rs for hardener and 220 Rs for Thinner (Liter).
You can do it easily, just show them the colour of bumber.
For the rest follow the steps mentioned above by you. You only mismatched the colour.