I'm very new to NSMB editing. I've exported the grassland graphics and just wanted to add some new tiles and import that, but it seems that no matter what I do if I import a bitmap it breaks the other palette used by that tileset and the colors get all messed up. What am I doing wrong?

Try exporting your original tileset by using the "Export all bitmaps will all palettes" at the bottom, and edit the newly exported tileset. When done, go back to NSMBe and click "Import all bitmaps with all palettes" and select your newly edited tileset.

Also, a screenshot would be more helpful
_________________________