Sadly it’s not possible (yet) to paint the terrain.
An (unsatisfying) option is to place a platform and add the grass texture
Roadmap says it’s ‘planned’ for end of this year but the roadmap is already in delay, so no one knows when and if it’s possible.
I don’t know what they mean by ‘Expand… terrain editing feature’ since this is the first mention of the word ‘terrain’ in the roadmap and later they even write ‘Begin development of terrain editing features’
