Hi everyone, I've managed to change the black and white palette of GB games into the green one that is used by Lameboy.
If someone wants to try to change the palette here's how I have done it: there is a section in the file gbgfx.cpp (in the folder arm9/source) that goes from line 582 to line 605 where the values of the color for both the sprites and the background are initialized. From my understanding one color is made up by two variables so, for example, you can change the first background color by changing the combination of the variables bgPaletteData 0 and 1. In order to generate the desired color you need to convert it from the most used 24 bit notation to the one used by gameboy color and nintendo ds: 15 bit. A useful tool is the one wrote by budmelvin, just google budmelvin 15 bit converter (I'm not allowed to post links).
However, if you use that tool you may need to write the last 2 digits of the color code before the first 2 because it seems that the order used in the gbgfx.cpp file is the little-endian. (Quick example: if you want to write the color 0x6BFC you need to set the variables bgPaletteData[0] to 0xfc and bgPaletteData[1] to 0x6b).
Once you have chosen the desired palette open the terminal, move to the Gameyob folder and type make to make the nds file (you need to have devkitARM installed).
So far I haven’t found any issue caused by the changes that I’ve made, but I’m certainly not an expert and I don’t guarantee anything.
Anyway, here is the compressed file with both the gameyob.nds file with the green palette for the Game boy games and the folder with the files used to built it.