Welcome to both you and Marvin!!! It can take a while for these guys to settle in. His color looks pretty normal to me for just being moved to his new home. Give him a few days. It can take up to two weeks for them to fully get comfortable in a new environment. I’ll attach some photos of my guy, he generally shows dark colors during the day especially while basking, that are pretty similar to what Marvin is showing. Don’t mind his shed in the picture haha!
I would like to suggest replacing the fake plants with live ones however. Veiled chameleons love to munch on plants and plastic ones can cause impaction. He may not seem interested in them now, but when he gets used to his new home, that will likely change. You can still use them for coverage if you like, just attach to the outside of his enclosure. That’s what I do and my Cham loves hiding behind them

I’ll also attach a list of Cham safe plants, just pay attention to which ones are marked ‘veiled tested’. Those are the ones you want.