Board Thread:Questions and Answers/@comment-71.91.113.48-20160116222736/@comment-1028741-20160116233232
No, I'm pretty sure purchased paintjobs should be permanently unlocked on your account, regardless of if you have since bought Horizons. As long as you're still on the same Frontier account and are using the correct ship class the paintjob is for, it should be there. If it's not, I'd recommend opening a ticket with Frontier's support team. The link is near the top of the launcher.