Thanks! Glad you like it :)
I don’t know how the GB Studio engine (or the Game Boy Color itself for that matter) handles colour so I can’t make any guarantees that it will work as expected with that mode, and updating it to support that is definitely out of scope.
If you have the plugin, you’re more than welcome to dig into it and make changes!
If you do, the plugin copies tiles from the background to an area just off the screen to build the view (ROM to VRAM) and then does a big copy of that constructed view to the screen (VRAM to VRAM). See the docs. It does this using “set_bkg_submap” and “vmemcopy” (in v4) respectively. After a quick look at gbdk (which the GB Studio engine is programmed with) it looks like there’s also a separate flag to set if you want to copy the tiles OR the colour data (attributes), the VBK_REG flag.
I don’t know if this means that each function in the plugin would have to be run twice to get the tile data and the colour data, or what the actual address in VRAM the colour info is supposed to be written (or even if that gets handled automatically by gbdk). Additionally, some of the underlying engine code uses 8 bit integers for tiles, meaning 0-255, so I don’t know if my plugin could reach the tiles above 255.
These are the things that first come to mind. Good luck!