Can't say I'm familiar with the story, but I'm basing mine from this quote in Richard's answer.
"artist had painted the sky the wrong colour and covered it with
blotches in an attempt, seemingly, to hide his mistake; the
perspective, what there was of it, was wrong; and the vegetation would
not have been found in the wildest nightmare. The whole thing was a
surrealistic portrait of hell"
While it could be practically anything, given Pratchetts humor and education it mostly likely is an actual painting of some description. Many impressionists painters would often exaggerate the colours in a scene to make a distorted view.
Below is The Red Vineyards by Van Gogh, which from someone unfamiliar with art may think absurd.
People labouring in fields of fire under a yellow sky with a nearly "green sun".

Is it this painting? Probably not, and without word from Pratchett it would be hard to say so. But it is likely Pratchett based the Alien's opinion on an impressionist or abstract painting of some sort.