The graphics are very stylish and no, they aren't monochrome
Actually bright and non bright are used which is a nice touch andmakes everything similar to original Game Boy even more.
The race parts are a bit worse with a noticable sprite flicker in car race. I wonder if it could be improved.
It's not easy but I enjoyed it with snapshots. Don't be afraid to do so if you had to repeat the same part again and again.
Also remember that you can float in the air and crouch to avoid bullets. It's useful in many places.
EDIT: For me it's wrong that if you die you have to start each level from the beginning. You should be starting from the current screen.