It's a big deal
The name Mark Ruthko is so distinguished in modern art, he did what McDonald's did to french fries. He owned abstraction. A room named after him on Level 3 of the Tate Modern hangs some of his Seagram murals – a total of 600 square feet murals painted from 1958 for an exclusive room in the new Four Seasons restaurant at the Seagram building in New York. You might recognise his work as two (or a few more) panels of colour with super soft uneven edges. 'Black on Maroon' was the target. A visitor calmly stepped over the wire barrier, pulled out a paintbrush loaded with blank paint (we are unable to distinguish an acrylic or oil property) and wrote the following: "Vladimir Umanets, A potential Piece of Yellowism"
Self-titled 'Yellowists' are potential nobodies
It appears there's only two of them. This one, Vladimir Umanets, is a Russian born artist who is not trying to strike vandalism, but draw attention to his 'movement'. Their website states it is not art and the rest is mystifying. Something about reducing interpretations of art to one: equalised and flattened. Yellowism pieces are not yellow.
Tate Modern closed for a period
While police investigated the scene.
Reactions: shock, outrage, upset, bemusement
It's silly and stupid so you can laugh because it can be restored. But people are angry at Vladimir's apparent satisfaction, believing he's added value to the piece.
But, it's not a rare thing
People attempt to deface artwork all the time. From Picasso's 'Woman in a Red Armchair' at Houston's Menil Collection earlier this year, to 'Mona Lisa' (well the box) continuously and Rembrandt's 'Night Watch'. See the BBC's list here.
Update point 6: He's been arrested.