In the overworld Editor, click
There, you check "enable custom palatte..."
As you can see, on the left side, the palettes 0-3 are unused.
So just add a snow palatte in 3, a desert palatte in 2 and so on.
Than in the 8x8 editor
you just select your normal gras tile and change the palatte to 3 or 2.
If you use this, you'll also have to go to the overworld menu:
Overworld --> Extra Options
And check the last box, otherwise the colors will glitch up.
A second option would be to edit on of the four GFX files and make new tiles that use different palattes.
This method is way harder, but the result will be prettier since you can leave the Extra Option part unchecked
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