The FJ40 color situation is genuinely one of the harder rabbit holes to navigate. What you're seeing in that listing photo as "Slate Gray" is almost certainly a mix of lighting, age-fading, and maybe some custom work along the way. Toyota didn't officially call anything Slate Gray in the FJ40 lineup — that color name gets used loosely by sellers and restorers to describe anything that photographs with a cool gray-green cast.
If you want that greenish warmth over straight Heath Gray, it's worth pulling the FJ40 color chart thread here on the forum (someone posted a full Toyota paint code chart a few years back). The code 616 Dark Green and the 717/Gray series sometimes look wildly different in photos versus real daylight. Your best bet honestly is to get chip samples from a local Toyota dealer's historical color reference or contact a shop that does Toyota vintage restorations — they often have actual painted panels on the wall you can look at side by side.
I've done a couple of FJ40 interior and exterior restores and the photography problem with these old Toyota grays and gray-greens is constant. The same vehicle can look completely different color depending on whether it's shot in shade or afternoon sun. If you can find a car in person that matches what you saw in that photo, get the paint code off the firewall tag and cross-reference it. That's usually the most reliable path when you're chasing a specific look.