Not to mention The Castlevania Games. Sure they had complete versions remade on the Japanese Collections, But no one has done the original 2 games from scratch.
The Konami Collections you mean?
Those are from Europe iirc, not Japan.
Both Castlevania Adventure and Castlevania II: Belmont's Revenge have full-coloured versions of them.
Only Castlevania which doesn't have a coloured version is Castlevania Legends, that one really lacks in that regard, but the other 2 are superb. I recommend you get the Konami Collections Vol 1 and 4, and then try the Autoboot hacks for both Castlevanias, so that you can have standalone ROMs of the two.
As far as coloured hacks go for GB games, I would love to see these ones:
- Super Mario Land - A proper coloured hack for it. Or at least colour the title screen for the current one

- Kirby's Dream Land
- Kirby's Dream Land 2
I feel like Kirby's Dream Land would REALLY benefit from a fully coloured hack.
Whoever decides to tackle that, you can try grabbing the full remake of it from Super Star Ultra for the colour choices.
I can't wait for the full release of this Metroid II hack for sure

Has some really nice colour choices which makes it shine over the other ones.