Muahahaha, you activated my trap card. About one year ago I researched a lot about the Gameboy Color because it's a part of the Gameboy that doesn't get a lot of love and you don't find too much about cool games besides the known stuff. I will give some recommendations but there is of course more. Prepare for a long post.
Personally I use the term hidden gem very sparely because of YouTuber using it for games that are OK but nothing to special, bought some games from Metal Jesus hidden gems until I realized that.
Here are some cool Gameboy Color games:
Baby Felix Halloween : Good Jump n Run with outstanding music and nice graphics. Sadly a bit short and with a lot of variety, so a nice and easy experience. PAL only and not easy to find even as loose cartridge.
Action Man Search for Base X: Cool Action Platformer that is hard but with the difficult settings you get different experiences because enemy placement is different. Made by Natsume so you get the typical music style it's known for, there made more action platformer that are mostly licensed stuff on the gameboy color.
Hands of time: Unique Zelda like with humor and mind blowing ending. Fun gameplay with a good mix of puzzles and action. Very good soundtrack so it's even fun when you don't know what to do.
Hunter X Hunter: Kindan No Hihou: Japan Only Action Platformer that plays really similar to a Castlevania game, not surprising it was made by Konami. Some describe it as a metroidvania but I wouldnt give it that genre really. Language barrier is there but with a walk through it can be done. Sadly cutscenes can't be skipped but otherwise I like the game. Not easy to find. When you like castlevania check it out.
Daikatana: Yeah, a Daikatana game that is actually good
. Its a action adventure and plays similar to Zelda stuff. Gameplay is quite fun and you do puzzle solving and some action stuff. PAL only and not too common but a bit more pricey.
Merlin: Fun run n gun, looks like a ultra cheap budget game but it's not. Good level design and difficulty curve. Best thing is that one of the special moves is a Kamehameha, absolutely funny and the reason why I bought the game xD. PAL only but not too rare.
Rhino Rumble: Very fun run n gun with one of the best soundtracks on the Gameboy Color. Story is goofy and you see that developers had fun making the game. You just do run n gun stuff with not that much variety but still very cool. One of the more pricey games but worth it.
Supercross Freestyle : Don't like 8 bit racing games but that's one I like. Very good driving feeling and very fun gameplay. You have also a Mario kart style mode with items, very fun. Sadly not that much content and replayability but worth it when you can find. Not expensive but quite rare and PAL only so good luck finding a copy and much more when you want it CiB.
The land before time: yeah, the game is more on the average side but why I still recommend it is because of the music. Holy moly, the music have no right to be such a banger for a licensed game. Easily one of the top 5 OSTs for the Gameboy Color.
Here are games that look cool but where I played just a bit but not too much, so I don't want give a recommendation for them: Alice in Wonderland, Blaster Master enemy below, Cyber Kuro-Chan 1 and 2, Lucky Luke Bandit Express, Moomins Tale ( Japanese version has a lot more content, PAL version is more streamlined), Return of the Ninja, Power Rangers Light speed rescue and time force (both from Natsume), Sabrina Spooked and Zapped, SWIV, the grinch