Set the emulator to SGB mode and turn off GBC mode before starting the game (press R before launching the game to see the other menus). That's all you can do unfortunately.

If you're looking for colorizations like the GBC can do, this cannot be done. The color info is actually stored on the GBC BIOS itself, with each individual game having its own entry. The GBC BIOS has not been dumped and therefore the only way to colorize it would be to manually create your own database of color data for various games, which would be a huge undertaking.

 

Like I said, Lameboy has no way of knowing what the GBC colorizations are. Therefore, you have three options:
1. Four-shades-of-gray palette, which I believe is the default.
2. Yellow palette, which looks like a GB screen. Choose it in the Lameboy menu before starting the game.
3. If you're lucky, a game will include Super Gameboy colors (and these ARE in the rom itself), so turn off GBC mode and make sure SGB mode is enabled before you launch the game. I know this works for Pokemon Red, most likely Blue as well. It won't be the same as the GBC colors, and it'll still be limited to 4 colors overall, but at least it's better than nothing.

Actually, looking at Red it appears since it cannot use GBC mode it automatically uses Super Gameboy colors by default. It should do the same for Blue.

 

1 strange thing.
some games actually turn greyscale on SGB while they are color in gbc mode.