Answer:
There was no question that John's painting, a huge colourful and ugly mural, was the worst entry in the art exhibit.
Step-by-step explanation:
Add a comma after painting and after mural. Notice how we are removing the description of John's painting from the overall sentence, but still allows the sentence to flow.
There was no question that John's painting was the worst entry in the art exhibit.
There was no question that John's painting, a huge colourful and ugly mural, was the worst entry in the art exhibit.
We do not want to separate the object and the statement by a descriptor without using commas.
Cheers.