Did I tell this person wrong?

Someone called to complain my company's website wasn't working. He said it was only getting a blank screen for two days. I pulled it up and it worked perfectly.

It has advanced JavaScript that allows thick diagonal color lines to fade in and out/ change as you scroll down, among other things.

I didn't make the site, but based on what I know about how settings and etc on individual computers can cause the end result of a website to be out of the programmer's/designer's control and the fact it was working perfectly on two office computer and my iPhone, I suggested to him it the code wasn't compatible with his computer.

He didn't say anything and continued to ask me another unrelated question. Once I helped him, he quickly said I was wrong about the website before hanging up.

Was I wrong? Or was I on the right track but didn't explain it clearly?

You're wrong to tell a customer it's his fault then not look for a solution. The site should be programmed in a manner that compensates for issues with older browsers. Step one should have been determining the browser he was using and whether or not he has JavaScript disabled. If possible have him upgrade his browser or test in a browser you know you support. For the sake of user experience you will likely have to sacrifice a few old versions but you should still compensate for it and not just show a blank screen or a jacked up page. If it's not supported make sure it's clear to the user.

If he is using a modern browser with JS enabled you need to figure out the issue so work with him until it's resolved. Don't just say "it is your settings" especially if he didn't change any. Find out any changes that went live the day he started experiencing the issues. If it's on your end figure out who made them. You NEED to know or the problem may snowball.

Yup. You're wrong, lol. It's probably a caching issue - at least that's the first thing you try. You have them empty the cache for their browser, shut it down and open it again. Sometimes I make them reboot.

An alternative is to have them try with another browser, or better - another computer.

The idea is not to tell them they're wrong, but to kind of prove it to them in a helpful way.