What are the real Game Boy Pocket "colors"?

Hello everyone!

Are the GBP "colors" (the four shades of grey) transposable to RVB system with a safe system/program? (I mean: not by just looking a GPB screen) And, if "yes", what are these colors?

I'm ripping all the maps from Link's Awakening and I want to give them the real colors (if they exist in RVB mode).

Thanks!

(French here, apologies for mistakes)