Think about two pieces of artwork.
Does the answer to any of these questions make you like one painting more than the other? This is why we compare art. To try and understand what the artist is trying to communicate through their work.
You need to look at the following items in both paintings.
Here is an example you can view online about comparing pieces of artwork.
'Fishing boats, Key West' by Winslow Homer (watercolor)
'Surf, Isles of Shoals' by Childe Hassam (Oil on canvas)
'The Kearsarge at Boulogne' by Edouard Manet (Oil on canvas)
A famous painting by Johannes Vermeer titled 'Meisje met de parel' (girl with a Pearl Earring) has had many versions of the painting made over the years. Compare the different versions and make sure to note the differences.
Original (oil on canvas)
'Girl with the pearl earring' by Lazaros Tsaktsiras (watercolor)
'Girl with the pearl earring' by elldog00