54Colour

Good news: you don't need colour theory. You need a couple of rules.

One accent. Pick a single bright colour and use it for what matters: buttons, links, important numbers. Everything else is shades of grey, or near-black and near-white. One accent looks confident; five looks like a funfair.

Dark or light - decide up front. And don't mix. Dark looks more expensive for most things right now, but it demands care: bright colours burn the eyes on black, so they need muting.

Contrast beats beauty. Light grey text on white looks stylish in a mockup and is unreadable in sunlight. When in doubt, more contrast.

Where to get the colour itself: steal from something you like. Open a site that appeals to you, screenshot it, ask the AI to pull the palette out. That's legal and everybody does it.