I have a solution for those who have had this achievement glitch on them. Everyone seems to be saying it works.
1. Sign out of your main account
2. Sign into an alt account
3. Start a public online match
4. During the match, press the Xbox button and sign in your main account
5. Go back to Rocket League and you should see a message saying "Primary User has no controller". Select the "Quit" option.
6. You should be taken back to the title screen. Press any button to go to the main menu.
7. You will be signed in on your main account in Rocket League and hopefully the achievement will unlock.
I unlocked a painted item and the achievement did not unlock. So I tried equipping the item and completing an online match and still nothing. Then I tried restarting my console and doing the same thing again and still nothing.
Due to the 10 hour limit on earning items, I farmed items on an alt account, trying to get a painted item but had no luck. So I did exactly what was mentioned in the solution by accident and the achievement finally unlocked.
I hope this solution helps you get your achievement.