If you open VBA's "Palette Viewer", you can notice the palettes are loaded to ram starting at 0x05000000. I might be able to help you out better if I knew what the palette map would look like in your hack but in original FireRed, it looks like the battle background palette starts at 0x05000042 (I noticed this by checking how the palette ram was affected in the first rival battle in Oak's lab when the palettes turn gray during Oak's speech.
So somewhere in that region, probably before 0x050000C0 (where some black palette data is located, again in original FireRed) you have some tiles using their palette as 0x7FFF (FF 7F) or values similar to that, leading into the white screen being part of battle background.
You should somehow debug how those palettes are loaded and by modifying some routines, change them into similar values that represent the new battle background you have got there.
And this job is far from impossible but it is actual rom hacking ;)
Hmm.... Mine starts at 0x05000042... :/
It seems I just replaced the wrong battle background I guess cause when I look in the Map viewer those white background was beside it.
I replaced the sprite #3 in UNLZ so is it the right sprite to replace in UNLZ for the grass background?