Great work, so far.
When you finish with Dr. Mario, if you decide to move on to other titles, may I suggest Donkey Kong 94?
There's a game that would be great in full color.
I agree. That game looks great on the Super Game Boy, but on a Game Boy Color, with even more and better colors, it will look fantastic. By the way, on the screenshots you had, I like the first one, but with the black background for Dr. Mario's area that's on the second screenshot. After I finish with the virus and Dr. Mario sprites, I can take a look at the clipboard and bottle colors as well as the water screens with the fish.
EDIT: I finished up the virus sprites. As for Dr. Mario, I tried to work on remaking him to look more like the NES version, but it just didn't look right, so I just did some very minor changes to the Game Boy sprites and gave it colors that fit the NES version. I think it will look good and fit in properly like this. I did try to give him his stethoscope, but I couldn't make it fit cleanly on the sprite due to the size, so I left it out. I might try again on adding it in though another time. He should be in a white background as that is the only color I can think of that would look best for this sprite. As for the viruses, I am not 100% happy with how the yellow one turned out, but I think it should do just fine. I might go back to it and touch it up, maybe. I also added the pills, tiny viruses, and the blink image when a virus vanishes, which I changed to match the NES version, but colored black to fit the white background. Now, to get started on the clipboard and bottle.