Not sure if this bug was reported already, but the 1UP Mushrooms in the claw bonus game (one of which is the rightmost mushroom seen in the screenshot below) are red instead of green.
Also, a suggestion. Mario Zone as seen from the main map could use more color. I came up with one possible solution using the existing 4-color palettes from the main map, which can be seen on the right side of the image below:
The center tile of Mario's cap, as well as the row of tiles containing his shirt, use the same white/green/red as the mushroom house level and giant turtle's shell (palette 2). As this palette doesn't have the brown needed for the cliffside, I used dithering to try to mimic the color. The tiles containing Mario's shoes use the light brown/green/darker brown that the rocks use (palette 1). These changes allow the normal green clifftop tiles to be used again without looking weird.
Yeah I highly agree with you on that one. It looked odd without color...
Also did you notice the second Mushrooms tile is mirrored in the crane game?
As people are talking about a Super Mario Land 1 or 3 for the Gameboy Color.
I think SML3: Wario Land should get the color treatment.
Super Mario Land would just be a plain Game Boy Color port of the original game with barley any changes to do. But as Midna says redrawing the graphics would be worth doing but it would take away from the original style and besides there is already a color version on the site anyways. It would just be a small colored version of Super Mario Bros clone on the Gameboy with new content the graphics and physics were very compressed.
Wario Land 1 would be kinda worth doing as that is the only Wario Land game with any color at all not even on another platformer it played more differently then it's sequels did. Wario Land II started as a Gameboy game but then it got a Gameboy Color port a year later with no other changes at all. Wario Land 3 was always a Gameboy Color game. As Super Mario Land 2 was the best game to get the color makeover I think SML3: Wario Land deserves it secondly.
Kirby's Dreamland and Dr. Mario would be interesting as they would look unique in color.