If you've completed the Ravensburger "Krypt" solid-color puzzle and need something of similar challenge, check out the insane "1000 Changing Colors" puzzle designed by artist Clemens Habicht. It's printed with a lenticular lens to create an iridescent/holographic effect where the colors change at different angles.

The effect is gorgeous to look at...

...but all the more challenging for it, as explained by Redditor u/puzzlecolornerd:

In case anyone is curious, it was definitely challenging but two things made it doable: first, the holo effect is made using lenticular imaging, which uses parallel tiny tubular lenses for lack of a better word so that you see the different colors at different angles, and that gives the entire surface a consistent grain which I could use to orient the pieces. Secondly, every piece was two colors, and along a gradient, so I could sort by the color pairs and find all the green/magenta pieces and separate them from the green/red or blue/yellow, etc.

I do not recommend gifting this to anyone who is colorblind unless they’ve wronged you in some extreme manner.

You can get the "1000 Changing Colors" puzzle for $89 on Amazon. Also worthy of attention are Habicht's similarly vibrant "1000 Colors", "1000 Vibrating Colors", and "1000 Halftone Colors" puzzles.