Source and Credits

Pokecommunity Post by Knizz 玉虫エリカ and DarkLight helped me to understand how it worked

Always make a backup before making changes!

How To

This feature does not work by default in HUBOL like the long and micro grass do ⁠(See Guide 2.8)⁠. Instructions for FR and HUBOL are the same

Tiles/block for the ash grass has to be drawn in (I copied them from Emerald)

Sprite for the ash grass "puff" animation is in graphics.overworld.sprites2 at Index 6

Change 59F50 from 00 20 70 47 to 24 28 F3 E7

Create your ash grass block and note the Block ID for it. (im using 556 for this example)

Convert the Block ID to hex (556 = 22C)

Go to 6EBDC and change 0A 02 to your hex converted Block ID in little endian, note this is a 2 byte value (22C = 02 2C -> 2C 02)

Go to 6EC00 and change 06 02 to the same value as previous step.

Create your grass block without ash, or just use the existing tall grass block. Note the Block ID for it (Im using the existing tall grass block for this example, Block ID 13)

Convert the Block ID to hex (13 = D)

Go to 6EBE0 and change 12 02 to your hex converted Block ID in little endian, note this is a 2 byte value (D = 00 0D -> 0D 00)

Apply behavior Unused_24 to your ash grass block. When stepped on it will convert into your block without ash using behavior byte 4 (tall grass) automatically

On any map you are using the ash grass in create a Resume map script with the following script:

changewalktile 1
end

The animation sprite exists already but the colors for it do not. Go to graphics.overworld.palettes2 and select palette ID 1005

You will notice there are 2 black spaces, these are the colors used in the animation

The colors from emerald are 948362 and B4AC94

Happy Hacking!