For some reason using an emulator, the game is not in colour like it is in the screenshot and web player. Did you provide the correct rom?
Viewing post in Murder Mansion (GB-Studio Game) comments
Yes, your ROM is correct one, the problem is your emulator.
All classic GB games are in black and white (plus 2 tones of grey), but with a GB Color, Super GB or an emulator you could use palettes to replace the B/W tones for other 4 colors. The green palette is pretty popular nowadays because it reminds of the original GB screen.
That feature is in your emulator preferences (it depends of which are you using). You'll have to pick each colors yourself in some of them.
BGB uses the green palette by default
mGBA: Tools->Settings->GameBoy
VBAM: Options->Gameboy->Configure...->Custom Colors
Retroarch: A GB Shader or something?