The default grass texture is not in the catalog, but it can easily be unlocked:

1. Open TSData/Res/Catalog/Scripts/terrainPaints.txt (make a backup first)

2. Edit the entries you want to unlock like so:

Before:

terrainPaint "lottexture-canvas-desert"
brushWidth 3
notInCatalog
terrainTypeSoundSuffix "grass"

After:

terrainPaint "lottexture-canvas-desert"
brushWidth 3
#notInCatalog
terrainTypeSoundSuffix "grass"

In other words, put a hash (#) before "notInCatalog", that's the delimitor for comments in these scripts.