Is it Jungle Quest? Completing levels gets you additional firepower at the start of each level.
If you don't want to play the game fullscreen, you can also use Magnifier to scale up what you see.
The game runs on 256 colours, so if you use a colour that's not one of the 256 colours, it will look weird.
You could try loading one of the original tile sets and using an eyedropper-like tool to copy a colour you like.
